I find it really strange that people who don't use lean don't just get on and use the alternatives rather that trying to get everyone who is using lean to use something else. It feels exactly like if all the emacs users in the world tried to force all vim users to use emacs.
It's important to meet reality head on: Every mathematician is not going to collaborate on the same tooling (as wonderful as that might seem on the surface to be as an outcome) human beings are different and want different things, and people are productive in different environments. In particular, people who want to formalize results within the standard framework (including zfc) are never really as a group going to care that much that lean4 doesn't let them formalize results outside of zfc.
It's important to meet reality head on: Every mathematician is not going to collaborate on the same tooling (as wonderful as that might seem on the surface to be as an outcome) human beings are different and want different things, and people are productive in different environments. In particular, people who want to formalize results within the standard framework (including zfc) are never really as a group going to care that much that lean4 doesn't let them formalize results outside of zfc.