Simon is having a bad start of the season, but he is a very underrated GK and he was one of the best in the World in the past few years, it goes unnoticed due to the club he is playing for.
GK like that will always have the trust of NT when he is performing well there, which Simons did fairly well for around half decade, he had the full trust of Lucho and LDF and it will take a lot to takeover his spot. NT coaches are always like that, they are usually loyal to their guys due to the nature of international football (they meet every month at best, or even few months).
Joan is only 4 years younger, it might take far more than people expect for him to be the starting GK for Spain, if it ever happens.