There’s a lot of empirical claims surrounding this topic, and I’m unaware who really has good evidence for them. The Substack guy e.g. is claiming that banning or demonetising would not “solve the problem” – how do we really know?

Well it depends what you define as "the problem".

If you define it as Nazis existing per se, banning them does not "solve the problem" of nazis existing. They will just go elsewhere. A whole world war was not enough to get rid of them.

However, allowing them on mainstream platforms does make their views more prevalent to mainstream users and some might fall for their propaganda similar to the way people get attracted to the Qanon nonsense. So if you define the problem as "Nazis gaining attention" then yeah sure. It certainly does "solve the problem" to some degree. And I think this is the main problem these days (even in the Netherlands which is a fairly down to earth country, the fascists gained 24% of the votes in the last election!)

However however you define "the problem" making money off nazi propaganda is just simply very very bad form. And will lead to many mainstream users bugging out, and rightly so.