I agree that arenas aren't always the best choice, but I think you're underestimating how often they are. Not only does a lot of software have elements that are well suited to arenas (request handling for web servers, frames for games and UI, etc), but a lot of allocations are for short lived intermediary values anyway (the stack is basically an arena with a bit less flexibility).

(Also, you do not need to sell me on bumpalo, I use it heavily already)