> so maybe with slightly different distribution of land use but basically fine and not necessarily as different as today is from 50 years ago?

No, probably very much more different than that, more like rolling back on industrialisation and globalisation. Closer to 500 years than 50, without the same hope of "progress" that we had back then.

> "mass extinction" already been happening for ages for many species.

Yeah, we all learned about dinosaurs when we were little kids, but if humanity collapses there's no guarantee of anything similar developing after us.