If your goal is to truly improve the state of the art in the ecosystem, dropping anything that is even remotely insecure is appealing I get that and I do believe the people behind BearSSL would love to do that. However to truly improve anything you need two things: Popularity and improve security.

There is a conflict there because popularity requires, at least some, compatibility to what already exists. You need to balance out security and compatibility. I think there is room for discussion about where precisely that balance is. You could further tilt it towards security by helping users of the library get a sense of what they need to support. Ultimately though you can't just blindly drop everything that's somehow not perfectly secure. Doing so would not improve security at all.

It's a small sacrifice to have one library be a little bit less secure than it could be, if that helps to make everything more secure it all.