Suppose a copyright holder knows people are infringing their copyright and chooses to turn a blind eye to it - and then after many years of doing this, suddenly decides to assert its rights and sue the infringers (maybe the copyright has been acquired by a new owner who is desperate to make money from it, somehow, anyhow.) Is it possible that the defendants might raise estoppel as a defence? Or argue that the copyright holders failure to act on the infringement when it was widespread public knowledge constituted an implicit license?

I’m not saying those defences would actually work. Indeed, my non-lawyer gut-feel is they probably wouldn’t. But now I’m wondering if anyone has ever tried those arguments? And if nobody has ever tried them, I think there remains a chance they might succeed, even if that chance is small. And if they ever did succeed, that would effectively amount to “defend it or lose it”, although the owner technically would not have lost the copyright, just (some of) their ability to sue infringers.

> suggests that BYU asserts the opposite

The page you've linked says: "Copyright protection is effectively never lost, unless explicitly given away or the copyright has expired. However, if you do not actively defend your copyright, there may be broader unauthorized uses than you would like."

How on earth did you read that and come to the conclusion that BYU 'asserts (sic) the opposite' ?