> Is it not an existential threat that large swaths of the population will essentially become redundant in the next decades?

This is precisely the crux. In the scenario where there's no mass-paperclipping etc, AI is not an existential threat. It's a threat to the current regime, and I probably agree with you that the regimes it enables will be worse for people with our current values, but it's not an existential threat to humanity.

The people who worry about AI existential threats primarily worry about one thing: can humanity as a species survive? There are likely scenarios where it won't. Those scenarios should be prevented.

Another frame to think about this is: a libertarian probably wouldn't be concerned about the threats described in your comment. But both a libertarian and a socialist will be worried about paperclipping.