I think you're right that nothing is a guarantee, at the same time assuming the future of Marvel movies is rather a dead end is a big assumption that I don't agree with.
First off you're thinking that the MCU was a much more solid foundation than it actually was. Iron Man is a certifiable classic, one of the best superhero movies in all sincerity. Robert Downey Jr was amazingly charismatic and the script was sharp and funny. But think about how much was abandoned. Remember how Iron Man 1 was building up the Ten Rings Terrorist Organization with the Mandarin being this big arch nemesis? They ended up abandoning that shit when Disney/Marvel realized that making a chinese guy the big bad might elicit a negative response from China. Iron Man 3 reduced The Mandarin into a forgettable comic relief. Frankly Iron Man 2 and 3 were rather forgettable movies. Thor 1 was a big dumb movie that everyone pretty much forgot, Captain America was a nice little scrappy origin story that set up Chris Evans as a great protagonist but held little weight beyond that. They turned Cap's main comic villian Red Skull into a cameo for Infinity War because the top execs said "Wait a minute, do we really want a Not-Nazi as a central figure?"
While Thanos is still an iconic figure among the hardcore Marvel fans he really wasn't well known by your normie popcorn viewers. And Marvel retooled him turning him into a Malthusian anti-hero instead of a man who was in love with the personification of Death because that would be really complicated to explain to in a fast paced fashion. Phase Two is honestly my least favorite part of the MCU, while Captain America 2 and Guardians are fun movies the rest are really forgettable.
Phase 4 has some real potential to excite audiences because Fox was holding the keys to some iconic figures. The X-Men are still some of the most iconic characters, don't forget Logan was the first real R-Rated Superhero movie and was very well received like Joker. For villains they now have Magneto, Sentinels, and Galactus who I'm willing to bet is going to be the big villian of one of the Avengers movies. They also have another "Civil War" type fighting event to adapt with X-Men vs Avengers which I have to admit I'll be very excited to see.
You're right that it could go wrong, but I think the appeal of Marvel is still strong. At the end of the day I want to see Spider-Man fight Wolverine in a Live Action/CGI event. A crossover like that still makes me happy even when it was
done in the poorly aged Spider-Man cartoon. There's a big hole for Disney to fill with Iron Man and Captain America now out of the picture, but there's still a lot of stupidly fun capeshit to be had.
Theaters might be a thing of the past, but Disney wants Disney+ subscribers and people buying merchandise. Because of the current situation the future of the MCU might become smaller in scale but there's still a demand for it.