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.

Artists get hired at companies because companies have the technology that made the artists work profitable, starting from book printing (public performance -> book printing -> cinema -> tv -> internet, similar to drawing -> photo -> digital). At the Public Performance / Drawing Era artists were mostly poor low class rogues. The technology made them what they are now.

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.