If they were instituting this because they want to have greater control over drunk guests, then they would implement it immediately. Seriously, if these guests are so rowdy that they're causing you to change well established policies, then why wait a month and a half? Do they expect guests to hold off their rowdiness in the meantime?
No, this is an economic decision. And I get that, except for the fact that: a) It's really not polite to bait and switch the folks who have already paid for their cruises banking on the alcohol policy remaining the same and b) It's not actually going to allow them to be more competitive. A few months after my parents get off their
DCL cruise with me they're taking a Norwegian cruise which is offering them free alcohol for the duration.