Even if they didn't make the promise in writing before people bought, they sure did make it verbally. I'm sure there's something in their T&C generally disclaiming the purchaser's any rights to have anything at all, but regardless, this is shortsighted and bad PR. BIC can't rely on Hamilton for making all its money forever, and they are really big on promoting subscriptions because they need them to sell seats to less popular show. I've been a subscriber for many seasons but I probably won't ever sign up again. I've also told numerous other people to subscribe because their customer service for subscribers has always been pretty good, but that definitely ends as well. Plus if I'm borderline on whether I want to see any shows (such as touring productions I've already seen in NY), I may just skip them. And I'm sure I'm not the only one.
Maybe this will make non-subscribers happy because they'll have more access to tickets, but they're alienating the people they really need to buy tickets to the many, many shows they bring in that aren't Hamilton, the Lion King, or the other sell outs.