I agree with one of your points (that where Figma really succeeded is that it understood better than anyone else the collaborative nature of web design), but I disagree that flat design vs. skeuomorphic makes much of a difference. I've seen more "realistic" designs where the actual iconography/images are created in something like Photoshop and then just imported to Figma, and Figma handles it fine.

Point being that I think that even if all apps still had the design aesthetic of 2008 iPhone apps that Figma still would have succeeded.