This is "If it ain't broke, don't fix it" kind of situation. A network-scale change won't happen without a very good reason, and I think most developers don't really care about key type or message syntax.
They simply want to connect their software to Mastodon. The real problem is a deluge of questions about HTTP signatures - I see new posts about failed verification on SocialHub and in fedi every week or so. This is what needs to be fixed, and the solution is twofold:
- Better testing tools: https://verify.funfedi.dev/ and others
- Documentation (like FEP-e2ce but ideally without proposing anything new)