It's probably okay to solve one problem at a time: first solve the "free open source browser, developed from the Web standard specs" problem in an established language (C++), and then the "reimplement all of part of it in a more suitable (safer, higher productivity) language - yet to be devised - problem.

And Andreas Kling already proved the naysayers wrong when he showd that a new operating system and Web browser can be written entirely from scratch, the former not even using any standard libraries; so beware when you are inclined to say 'not feasible'.

Maybe? I feel like there's been lots of efforts to migrate large C++ codebases over the years, and few actually complete the migration. Heck, Google is even making Carbon to try to solve this.