A lot of extensions are made because people need them for themselves, not to live of extension development. And this is something that can exist as a nice open

A lot of extensions are made because people need them for themselves, not to live of extension development. And this is something that can exist as a nice open source ecosystem with some oversight, like packages on a typical linux distro. All the development stuff can go into something like AUR on Archlinux, where users have to explicitly participate and accept more risk. And this is something not very evil browser vendor might be willing and be able to do. But creating incentives for extension developers is definitely the opposite of their plans, they'd rather kill extensions altogether.