Unlike trademark, you do not lose your copyright registration for failure to enforce it. However, it makes it a lot easier for prospective defendants if you have a track record of ignoring infringements. Your registration is not in danger, but your ability to win as much as possible might be. Even works with no identifiable copyright owner are not necessarily considered 'abandoned,' which creates a lot of interesting issues, such as in the case of Google Books: https://scholarship.law.duke.edu/cgi/viewcontent.cgi?article...
The main thing with copyright is not like the risk of trademark dilution: it's just that not enforcing it makes it easier for defendants to bargain and argue for lighter penalties.