How will the MCU Age? no clue but the MCU is 12 years old with 23 films released so far and going strong with no flat out Cluster F of a film while look at Superman 1978, Batman 1989, and Spider-Man 2002 3 of the best Superhero films of all time.
Yet by 1987 11 years after the 1st film the Superman series released just 4 films and the latter 2 float between not good and flat out terrible.
Batman 89 had Batman and Robin released within 8 years of it's debut.
Only 5 years after 2002 Spidey we got one of the best comic book films Spider-Man 2 but then ended on the mess of Spider-Man 3 that even Raimi says is bad.
The X-Men franchises 12 years in only had 5 films 2 of which were bad Last Stand and Origins but had rebound in 2011 with First Class.
How will they be remembered down the road? They'll probably have a good shelf life due to a generation of kids growing up on them but once that generation gets past their nostalgia phase in about 30 years who knows but as for now they're doing better than the rest of the Superhero franchises that came before even ones that gave up some of the most iconic films in the genre.