To your last point, couldn't you just as easily point out that the Marvel films "get a boost" from a long history of published comic books that people might read - either current issues or back issues that are easily find in libraries as compiled graphic novels - or even the numerous "based on" books that are floating around? Or plenty of other media (e.g. various cartoons) based on the same characters. If the Marvel movies spark someone's interest, there plenty of related material they could engage in enjoying.
As to your main point, IMHO it just seems to be another way to underestimate the MCU as people have constantly done all along. I have no idea what the future holds, but I think the MCU films in general are entertaining, easy to watch movies. They are the type of movie that people will be flipping through channels and just stop on because they are fun just like many "summer blockbuster" type movies. (Now, with the rise of streaming and fewer people having linear TV, that specific phenomena may not happen as much but some similar thing will exist and Disney+ has an ever increasing subscription rate)
And I think these characters -- and the specific MCU version of them -- have achieved a certain cultural zeitgeist as to be well regarded and know for a long time. I don't really think there is much to fear abut n olonger being know or popular any time soon.
Not really, because comic books have a tiny percentage of the readership levels of Harry Potter (which I think is the highest selling book series in history). Additionally, Harry Potter (and Lord of the Rings) is a finished story with a beginning and end. Comics continue on indefinitely with continual reboots, resets, etc. so there's really no particular place to start or stop. In any case, I was really talking about the reverse -- that some people read HP or LOTR and are then directed towards the movies. It's especially helpful in HP, because the early books are written for elementary school aged readers and so it's easy to get them hooked early as a way to encourage reading. That's happened with all of my nieces and nephews, as well as several younger cousins.
In my experience (and it could certainly be wrong) the MCU doesn't have the level of hardcore fandom that something like Harry Potter does. There are tons of people who enjoy the MCU, but a much smaller percentage of them are totally engrossed in it compared to the percentage of HP fans totally engrossed in that series, and I think it's because the MCU isn't fundamentally escapist the way something like HP is for a number of reasons. There seems to be a much higher level of casual fandom. People absolutely freaked out over the ability to go to Hogsmeade and Diagon Alley because they got to enter that universe in an escapist way. Something like the Avengers campus doesn't have anywhere near that level of excitement, and again, I think a big part of that is the lack of (or at least lesser amount of) escapism.
I don't think I'm underestimating the MCU. I'm a big fan of it, as I said. I've seen almost all the movies and have seen most of them multiple times. My issue is really that even for the biggest properties, maintaining that level of interest for the long haul is exceptionally difficult and exceedingly rare. Just on simple statistics, the MCU is far more likely than not to lose relevance -- and I do mean the MCU specifically, as in the MCU incarnations of these characters. Not Captain America as a whole, but the specific Chris Evans Captain America.
As just one random example, the Back to the Future trilogy was massive in the 80s and are one of the highest grossing trilogies ever. Back to the Future made more money (inflation adjusted) than all but 4 MCU films. Ask anyone under the age of 30 if they've seen any of those movies and you're going to get an overwhelming amount of nos. Things just don't maintain the cultural relevance you'd expect and it's hard to see that in the moment. It's not a knock on the MCU; it's just a cultural reality.
I could be completely wrong, and we obviously have no way to know until the time has actually passed. It's just that if I was placing a bet, the odds would probably be in my favor.