The problem is if it becomes a normal word in everyday use, then it can lose it's trademark. I suspect it will happen to google soon, I definitely have heard people say they googled something on bing. Wikipedia has this list of trademarks that were lost this way: https://en.wikipedia.org/wiki/List_of_generic_and_genericize...

This issue with Notepad++ isn't even really about trademark, even though that would be the legal means to enforce it. It's more about Criteria #4 of the opensource definition "Integrity of The Author’s Source Code" Which is there so you can't give the original project a bad name while forking it. https://opensource.org/osd