This feels backwards—if the service gets worse, simply cancel your subscription.

It's not backwards. I've canceled many subscriptions because software has gotten worse. I occasionally check back in with those services to see if they've gotten better, or at least back to as good as they used to be and they are always, inevitably, worse.

I'd love to hear about a subscription that gets better over time but I haven't had that experience. I've also heard how product managers talk about subscribed users and I don't like being thought of that way by the businesses I give money to.