But there would be no basis to claim this trademark was abandoned (even before Don Ho responded to infringement). Notepad++ is famous software actively getting new features and new releases. It is well-known among technically sophisticated Windows users in the US, and until this kerfuffle Don Ho's ownership of the name was never seriously contested in OSS circles. Nobody could reasonably claim this trademark is stale or generic.

It's nuanced. I'm not an attorney and I don't have the bandwidth right now to go looking for citations. The general read I'm getting is that zealously defending your rights to the mark are the safest way to make sure a court doesn't see you as abandoning your rights. I gather that different courts in the US have treated a lack of defense differently.