So let's say The Marvels bombs big. The MCUs most appealing active character gets chucked in the trash. Great.
Then what? We know the TV slate and production process is getting revamped, which it needs. What of the films?
Captain America and Thunderbolts are risky in the current environment, but they're as good as in the can. Outside of extensive reshoots which are typical for Marvel - and I have no idea how you correct the current issues in reshoots - you don't have a lot of room to maneuver there. Deadpool 3 will be a hit - that's safe.
Now you're into the period when you have more options. Blade's a production mess, so maybe you kill it, but honestly a simple Blade vs vamps story seems like one of Marvel's surer bets, and it might be a good place to test a back-to-basics approach. The Avengers two-parter comes next. Marvel was almost certainly always going to bring back Downey and Evans here, so you beef up their parts and trust the Avengers label to bring in viewers. Maybe you combine the two films into one to limit your risk if you're really skittish.
Outside of Blade, your biggest question mark is The Fantastic Four. It's a property that's bombed big twice, and while I would have scoffed at that being an impediment to the MCU version a year ago, now I'm not so sure. For sentimental reasons, the MCU feels like it needs the First Family, but at this stage sentiment may not matter, Beyond that, you want Doom in the MCU - is that enough to take the risk?
As I see it, once you get past the two risky features you've already paid for, the MCU gets onto firmer ground and you can reassess post-Avengers plans. The question becomes - what of Blade and FF?