This is the crux, isn't it? I suspect that this is simply the early stages of boiling the frog. Establish the idea with some niceties alongside the offer of one-time payments and then, once people have got used to the idea, drop the option for one-time payments, pull more core stuff into it, and jack up the prices. I wouldn't be too surprised if this eventually leads to pay-to-drive subscriptions if it is allowed now.
And, yeah, you can call that a slippery slope argument if you like but I think it follows the pattern we've seen elsewhere. Companies love to replace one off payments with subscriptions.