It threatens because we aren’t just talking about selling your art. Artists get hired at companies to produce all kinds of work that will now be replaced by AI.
They are protesting against natural technology development. To me it looks similar to taxi drivers protesting against Uber (protecting their right to scam tourists).
Did drawing artists protest against photography? Do celebrities protest against photographers selling their photos taken by them in public places?
They are right to be afraid though. What's really happening here most probably is Anthorpic buys rights to collect user trajectory data. In order to replace Blender users later.