I'm a bit surprised you haven't seen more of a move towards annual plans. HBO already offers a 20% discount. At some price delta you start to make switching in and out unattractive for all but the hardest core binge watcher and optimizer.
I would like to see a breakdown of how long the average user stays subscribed but I guess it’s too sensitive to ever be communicated. I don’t think I have ever subscribed more than a month at a time every couple of years to watch something very specific (or play actually - last time was because of Immortality). While I think this is an unusual customer profile, I wouldn’t be surprised if short subscriptions still represented a significant number of accounts.
I'm sure it's a significant number in the aggregate. However, knowing people and subscriptions, I suspect a very significant majority keep services for years even when they're not using them much. Certainly, I rarely cancel and, if I do, it's probably going to take something I really want to watch to pick back up again. I also don't really binge watch so if something is a season or 2 it might take me a good 6 months to watch it anyway.
I find a couple things a month on HBO Max that keep me on the service anyway. I find it hard not to have Disney+ by comparison... pretty much can't get hulu, verizon or half a dozen other things I use(d) without Disney+, wonder how much they're cheating on their subscriber counts or how much they're actually making.