They'd already be getting tired but they're not part of national teams who compete for serious titles (firmino wasn't a mainstay until recently). Like MSN.
And they haven't won a lot yet. Just 1 CL.
Well, my opinion lately is that BOTH Pep and Klopp have a coaching idea where they are getting 120% out of players short term, for 2-3 years.
And players are dead and overburnt both physically and mentally after that.
Pep's City is falling apart both mentally and physically in this season, as always after 3 seasons.
Regarding Klopp, his team is also consisted of players who currently look, run, press and play like tireless robots, which is not sustainable longterm.
Also, you can create such teams (Pep's and Klopp's) only with players who are mostly young or in their mid 20s, and who haven't won a lot, so who's physical and mental batteries are full.
Imo, Pep will run away from City after this season, NOT because HE is tired or because HE needs a new challenge, but because he knows what he is doing and his team will be dead on all levels after this season and the only way to bring it back to life is to sell half of the drained players and to build a new team, which takes time, suffering in results and which brings fights with your current players whom you now need to sell in order to buy fresher players who will be able to give 120% for 3 years (A coach will look like an asshole or a selfish traitor then), or=to run away, which he is always doing.
Regarding Klopp, imo, he is doing the same trick.
Liverpool are on their prime for the last 18-24 Months.
They will win EPL in upcoming Months.
Next season will probably be way weaker, like Barca's 2007 under Rijkaard after a CL win in 2006, or Lucho's 2016 after treble.
They will start well but will get weaker and weaker with each Month passing by.
Again, Pep's or Klopp's players aren't ACTUALLY robots.
And what their coaches are doing with them currently can't last for long.
Anyway, when you sum it all:
1. there is a high chance that Mane, Salah and Firmino will suffer from accumulated fatigue (from playing 24-36 Months like robots at 120% without rest and rotations) and that they will play way less than today
2. injuries will start to strike them after playing like robots without rest for several seasons in a row
3. there is also a mental part: there is a high chance:
1) that some players will lose their motivation and turn into party mode after winning a legendary EPL after 30 years of waiting and after winning a CL in 2019 (like Ronaldinho and Deco after 2006)
2) or that they will need a new challenge (Salah/Firmino moving to Barca/Real)
So, imo, a current Liverpool is not sustainable.
The 4th guy could easily turn into a starter soon (longterm) due to all these reasons from above.
