I think the extremists are those who don’t follow science and educated thought with greed and avarice removed. We can’t continue with the current percentage of population so engaged with consumer culture.
More extreme than this, I believe many of the issues facing mankind are most feasibly managed by a massive reduction in human population.
Tempering this I’m not perfect myself regarding environmental footprint, and I regard myself as being more worthy of removal from the planet for the benefit of the environment than certainly most third or second world folk. I’m not actually volunteering mind you. I’m doing lots by not breeding.
It’s a disgrace to call Extinction Rebellion extremists, except in the very particular sense on their topic they are the most motivated to agitate.
Governments/society need to evolve democracy/consumerism/capitalism quickly or society will have very rough storms to weather.