Async support is a big deal for a lot of people though (and it's neovims original raison d'etre). For example, not having :make (well, :Neomake) block on compile is pretty significant by itself. Under the hood getting real async tasks required/requires a ton of rewriting because of how Vim was architected. Those changes were made deliberately and weren't just so that they could use all these fancy new libraries.
So I guess I'd liken it to Windows when it went from the 98 days to WinNT/2000 - keeping things working about the same while laying the foundation for bigger improvements later.
On the other hand, I don't see much downside to moving to neovim either. It is interchangeable (in my experience) with vanilla.
My vimrc is 80 lines, I don't feel it's vanilla at all, but I don't use :make or probably any other commands that launch subtasks so I guess this is what sets me apart.