What will keep Adobe working on improving and updating their software are things like Corel PaintShop Pro.
Adobe has been standing head and shoulders above any serious competition, and that in turn has put them in a near monopoly. However why would anyone assume that they're even dreaming of just sitting back and giving their competitors time to copy all their features and try and take the lead.
I've installed a dozen major updates across all the titles I've been playing with from CC in the few months I've had it, which have added some neat features that in the previous model I would have waited a year or two before the next 'major release' rolled out.
Subscription gives a stable, constant income that can be reasonably relied on. This means that they don't have to bank up as much money as they can from each major release cycle and hope that it holds them over for the development of the next. It makes business management so much easier, and companies more willing to take 'risks'.