As long as your customers are in the vast majority accessing your web apps over non-cellular data connections this makes sense. As more folks are cell-phone-first, ignoring the size of the payload you expect users to accept will increasingly represent a sub-optimal experience.

It's not a question of cellular connection quality but of data quantity on a plan.

But you're not the one implementing the frameworks. For those, being slim is business value.

also, page size is not the biggest performance killer. too many http requests (for example, requests out to CDNs and advertising networks) is a more likely source of problems.