Trademark law is pretty much "if doing this thing makes customers think they are buying products from another company, then you can't do it".
The reason you can lose trademarks is because, if you allow people to use it without contesting it, you lose the ability to argue that they would think they were buying from your company.
Trademarks are very very different from copyright law, as they are primarily intended to be an anti-fraud consumer-protection tool. A stand-in identifier for the company that owns them. This isn't always how they are used, but this is how they work.
Copyright in of itself has strong competent 'use it or lose it'. Your case against infringers (that you were aware of), weakens the longer you take to send a C&D. After a due time, when you can show that all parties were aware of what was happening, you can argue, and has been argue that you have a gratis to continue your infringing work. If I didnt have ~2hrs to be on line, I would go dig for citation, for this.
Even if that's true, for copyright at least, there's no general precedent. Letting one person do it does not weaken your case against anyone else. For trademark, letting someone else use
does.
Also, your cease and desist will still stand, it's just that you'll have a hard time suing them for previous use.