Real Madrid probably will always be at the top as they have been in a good part of the last century, I am not sure it will always be the case for us. We didn't become an internationally renown club until Nunez took control of the club and thanks largely to Cruyff, and later reinforced by Ronaldinho and co. and of course solidified and brought to an even elevated position by Messi and co. With deep pockets and seemingly unlimited resources, the likes of City and PSG will win European championships, it is just a matter of time. And if they will it several times they will soon reach the status of that of Real Madrid, us and Bayern etc. today. Not saying that we will become the likes of the Milan clubs any time soon but just look at how much clubs could fall. We have been doing great in the last 10 years or so influence-wise primarily because of Messi and once Messi is gone, I don't doubt that we will become less influential with a part of our fanbase going away with Messi. And if for whatever reason we don't do well on the pitch, sponsorship and other components of our revenue will likely take a hit.
The English league is the most popular league in the world for several good reasons. They are competitive with the big 6 (in Spain you have the big two or three at most), English language is far more influential than Spanish (we, fans from all over the world, are posting here in English) to the average non-English and non-Spanish speaker. The foothold and head start the English league have gained in the rest of the world vis-a-vis the Spanish league or the German league etc. is enormous and can be washed away in a few years. In many countries that I have been to, the English league is being broadcast on more popular and accessible channels and the Spanish, the German and the other European leagues are often found on some obscure channels or online streaming sites exclusively, no wonder their TV deal brings in so much more money than the other leagues.
I think more British folks live in Spain than vice versa is simply because Spain is a lot warmer and has much better weather over all than Britain. Not surprising. Probably the same case with British people living in southern France than French people living in the UK.
That is wrong. FC Barcelona was arguably the biggest club in the late 1940's and early 1950's. Read about the era of Kubala, Czibor and Kocsis and the "Barça de las Cinco Copas". We won the Latin Cup (the predecessor of the European Cup) twice (more than any other club) as well. We had more league and cup titles than RM for the first 50 years of both clubs existence. It only change after the famous and very controversial Di Stéfano case and the (for Barça) absolutely appalling 1960's and 1970's (Franco era) and partially 1980's where RM won 2-3 times as many league titles if not more while we won it 2 times, lol. Back then only the league winners played in the European Cup and no European club had such an powerful opponent like RM during the Franco era that routinely won the league with some very dodgy things going on that anyone familiar with the history of Spanish football will attest too. Having very bad boards (much like today) that helped sabotage us, despite our enormous potential, rich history, being BCN (one of the most beautiful cities in the world), having the largest stadium in Europe etc. did not help either in those for us very sad decades.
Even then we had many stars and world-renowned players like Cruyff and Maradona (most expensive players of their time) to name a few and we had European success too and won the Inter Cities Fairs Cup (predecessor of the UEFA cup), Cup Winners Cup (only second to the European Cup back then for a large time period) more than anyone else and several CdR back when that cup had a big significance or at least greater than nowadays. As well as the occasional league title and reaching European Cup finals and semifinals yet failing against inferior opponents in Benfica and Steaua Bucharest etc. Similar to how RM could not win the European Cup/CL for 32 years between 1966-1998. Spain also had much harsher VISA rules than the likes of Italy and others had which meant that elite players, who otherwise would have gone to Spain, went to Italy and elsewhere in Europe. That changed when those in power started thinking rationally again and stopped shooting themselves in the foot.
Now for the past 30 years (1/3 of our history roughly) no club has been close to us in terms of combined domestic and European success, number of trophies, having the most iconic players, having the most iconic playing style etc. We have turned into a powerhouse on every front and we won't be going anywhere soon if ever. You are way too pessimistic.
BTW, for half of their history, RM has been a far less succesfull and prestigious club than us so the idea of them "always" being at the top is wrong one. If anything I think that we will catch them in terms of La Liga titles within 15-20 years as well as close the gap in terms of CL titles or get a better start whenever that European Super League emerges.
That is guesswork for now. What I am only focusing about is the reality of today and here QSG and Man City are nowhere near the elite clubs. Being backed up by entire states or not. They might well one day have the prestige of the two giants (I would not include Bayern here with all due respect to them, they are a tier below) but it is far from given. We are already at the top and have stayed at the top for decades. That is the most difficult thing.
The myth of there being great competition in England compared to Spain is what it is. For all the talk about competition the same 2-3 clubs are always winning the league. Leicester being a freak result much like Deportivo was in Spain during the 1999-2000 season and Real Sociedad being close to doing the same (they lost the league to RM by 2 points) back in 2003. The EPL era was totally one-sided for the first 20 years, only interrupted by a lucky Blackburn win when the EPL was a joke (1990's) and the emergence of oil money when Abramovich bought Chelsea. If not for that it would be Man Utd winning 80% of the time (like they did for almost 15 years) and occasionally Arsenal winning once or twice.
You are underestimating Spanish severely. it is only second to English in terms of importance of European languages and has more native speakers than English and most importantly the most football obsessed continent in the world (South America and Latin America, I know that the latter is not a continent) speaks Spanish or Portuguese (Brazil) which is extremely close to Spanish as I personally understand almost everything in Portuguese (written and spoken) without ever being taught that language even for a second.
Fact of the matter is that the popularity of the Spanish league is on the rise everywhere, in particular in Asia and Africa (Spanish league is the most popular in the Arab world, 500 million people, for instance) and gaining foothold in East Asia and South East Asia.
Spain is simply a more beautiful country than England landscape wise (no comparison), in terms of historical heritage, World UNESCO sites, living standards (the good life if you got money) and weather. I would also dare to claim that locals and the people as a whole are more friendly and warm much like is the case in the remaining parts of Southern Europe. That the UK is a more industrialized country, has 15 million more people and a bigger economy (although neither Spain nor Italy are pushovers, in fact elite economies themselves worldwide) and wages are higher (although costs of living are higher too) is what it is.
Anyway as far as I see it the two Spanish giants are untouchable and if one day hell breaks lose Spain (a far more wealthy country than Qatar and UAE combined many times over) will do everything in their power to bail them out as both are important for Spanish cultural soft power in the sporting world and tourism as well as symbols of the two largest cities in the country.