You said: "and support expectations clear,"

Therein lies the rub. By not honouring the trademark, the fork made the association of service, support, otherwise to Notepad++ making it seem like it was officially supported.

Imagine if someone who used the fork attempt to get support on a product that wasn't supported and, when faced with limited responsiveness, etc. decided to denigrate the original developer by lambasting them on HackerNews, et al. The reputational damage alone would be seen as a reason to defend the mark.

I work with a very large OSS nonprofit who has trademarks in most of the geos around the world and vigourously defends them for precisely this reason: reputational damage undercuts the community, the developers, and the reason for existence.