> And Mermaid could have decided they want exclusive use of the name and trademarked it half a decade ago.
If you do not trademark your name then I can only assume you do not want exclusive control of that name - I certainly wouldn't assume you want the protections a trademark grants.
I'll cede it might be precarious but the whole point of having trademark protections you apply for is by definition contingent on not having those protections if you don't apply for them. If you don't apply for them and someone else uses the name too why should you care?
To me it sounds like when open source project resenting a very-permissive license just after someone else turns it into a profit machine. Kind of sucks but also the whole point of that permissive license is you shouldn't care.
"You didn't register your trademark, so I guess you don't care if I use it!" Setting aside mermaid here, that line of thinking is not only legally precarious but heavily unethical. In the US at least, trademarks are implicit, like copyright. Registering a trademark can help in court, but it's not required.