What makes you say you can't modify them? Are you referring to modifying your copy, or are you referring to modifying the main branch and thus other people's copies? You're trying to blame this on "megacorps". However, as a user, I appreciate the maintainers at Mozilla for keeping Firefox well-tested and well-maintained, part of which is scrutinizing feature proposal and PRs/patches to make sure that their end users would want them. This is such a strange criticism of Chromium/Firefox. At least you can modify your copy at all.
Your other comment suggests trying to add a new web standard but IMO the unending stream of web standards is why only "megacorps" maintain browser engines, because only they have the resources and care enough.
Either way, it's a logical leap to suggest you need to modify Chromium or Firefox to implement this in the first place. The concept from TFA can be implemented with webviews without seeing any code from any browser. To answer your first question, the only thing stopping any implementation is time.