The more I think about this, the more I think Marvel has more to lose from a deal not being made.
Spider-Man and related characters are Sony's only comic book IP, and it's IP that will make money no matter what happens. They'll no doubt get a little bit more money from being in the MCU, but not more than what they'd lose if they gave up too much of their share of the revenue to Marvel in any kind of a deal.
On the other hand, Marvel has a tonne of other IP which is otherwise way less popular than Spider-Man. Spider-Man is a multiplier on any money they make from any of the MCU movies, because people want to see an MCU that has Spider-Man in it. Thus, the benefits that Marvel gets from having Spider-Man in the MCU are not limited to their share of the Spider-Man movies, but are compounded across their other films.
This is particularly the case in this very moment in time, when Marvel is transitioning to the cast for the next 10 years. They're introducing a lot of new, fairly B-list characters to the MCU (forgive me, but I'd never heard of Shang-Chi or the Immortals), while a lot of cornerstones have moved on. Spider-Man is their most popular character, and would be not only critical to the storytelling of phases 4, 5 and 6, but to give continuity from a commercial perspective.
Knowing all of the above, it actually seems like a fairly bold gambit for Marvel to demand 50 per cent from Sony. It's a gambit which I expect them to back away from, because while fans may on the balance be blaming Sony at this moment in time, in the end they'll be voting with their feet.