The real answer is 'it depends,' there is no pat hard and fast answer about the obligation to enforce a trademark. Also it becomes more ambiguous because trademark cancellation proceedings are quite expensive in terms of time and legal fees. You can do a lot of naughty things and get away with it if the prospect of fighting you in a cancellation proceeding becomes too daunting and expensive. The possible workaround is to get a declaratory judgment invalidating the trademark but that is not necessarily something achievable in most situations.
The real answer is 'it depends,' there is no pat hard and fast answer about the obligation to enforce a trademark. Also it becomes more ambiguous because trademark cancellation proceedings are quite expensive in terms of time and legal fees. You can do a lot of naughty things and get away with it if the prospect of fighting you in a cancellation proceeding becomes too daunting and expensive. The possible workaround is to get a declaratory judgment invalidating the trademark but that is not necessarily something achievable in most situations.