This is also how corporate software subscriptions work. The idea is that software development is ongoing process (new features & security fixes) and that there are related costs you have to pay as well (support people, QA, infrastructure, etc.).

So it makes perfect sense to make also the licensing/subscription an ongoing process, to better reflect that & sustain the development and fund the related related costs. There is also the motivation factor for the company - the customers might not renew their subscriptions if they are not happy. So you do what you can to keep them happy & subscribed (new feature and RFE implementation, prompt bugfixing, fast handling of security issues, responsive support, robust infrastructure, etc.).

There are also some specific mobile device relevant aspect of subscriptions - it might help to finally fix the issue with manufacturers dropping support early for perfectly functional hardware. They normally get all/most of the money when you buy the device and don't really have any monetary incentive to keep supporting the device once it has been sold - they want you to buy a newer model so they will again get your money. An not releasing updates (AKA planed obsolescence) can only help with that.

Now consider the subscription model - you get money from users as long as they run yous OS, regardless of the hardware. So you don't really have any incentive to stop supporting devices as long as enough people who have them are paying. This might help breaking the current vicious circle of planed obsolescence.

Also, even though people might not really notice it, they are actually already paying a subscription for mobile OS. It's just they they are paying with their privacy and private data, instead of money. That's another area where subscription might help - you will just pay them upfront and they will not have an incentive to sell your privacy to the highest bidder, as that might make you drop your subscription.

I can see people asking why they should pay for OS if it they are normally getting it for free. But it' not actually free, they are just paying with they privacy & the need to regularly pay for a new device as the one (perfectly working one) they have is no longer supported and updated.