I cut the context because the context doesn't matter.

Anyone making the claim that this likely the last era of humanity that anything is just wrong. The future (even just the future of humanity) is longer and weirder and wilder and more filled with unknowns than anyone alive now can imagine.

This is not the last era of humanity that anything.

We should still be protesting/fighting unchecked power.

There have been many last eras of humanity of things. The last era of gas lighting (lighting powered by gas, not the manipulation tactic). The last era of information being passed down orally. The last era of monarchs. The last era of human computers. I understand that you are saying there are no last eras of anything so this can't be the last era of something, and that's incorrect.

The post I was replying to was not worded as "the last era of X". It was worded as "the last era of humanity". Certainly things, practices, places, peoples, beliefs come and go. Humanity likely will end at some point, but that's not about to happen in any predictable way in the currently foreseeable future.

Incorrect. It was worded as "the last era of humanity doing X". It parses as "the last era of {humanity doing X}" and not "{the last era of humanity} doing X" (which isn't something that makes sense anyway).