Depends. Which app does need complex animations? And you could very well build Twitter as a PWA (and probably FB). Maybe not Snapchat. Most apps aren't heavy on filters, camera and video.
That’s valid, I guess. I think that if a social network wants to take off today (especially amongst people under the age of 25), it’s going to need to be a little fancier than Twitter. But PWAs could replace a lot of the more basic applications that exist today.