Dude...it isn't an "opinion."

https://80.lv/articles/nexus-mods-owner-acquires-steamdb-after-co-founder-struggled-with-burnout

https://kotaku.com/nexus-mods-steamdb-steam-2000730486

https://www.techspot.com/news/113712-steamdb-acquired-nexus-mods-after-13-years-independent.html

Guy was even apparently looking for a replacement for himself for years before deciding to sell after burnout. So yes, saying it is because people did not donate isn't really consistent with the evidence and explicit reasoning the owner gave. You are not only expressing an opinion, but an unsubstantiated one.

Also, yes, it is hardly fair to say that people don't have a right to feel upset that a free service has the potential to no longer be free. In fact, what you've described, the need to spend money to do so, is antithetical to the philosophy behind that model. Chill out and adopt a more understanding perspective, it is seriously less work and more effective.