The problem with that model is that after owning the digital copy you have no native platform on which to play it. You always need to access it via the PS NOW service and part of their cost structure is providing the infrastructure that streams the game to you.
Yes and No. It's true that you'd have no native platform, and yes, part of the cost structure is providing rentals for those servers, BUT I think ultimately the service would bring in more money if they allowed you to purchase games and the rental side was just a gateway to that.
Think about Steam for a second, how many people have libraries of hundreds of games they don't play or never played again after the first playthrough, even if somebody does buy 100 games and wants to stream them fairly regularly, he'll never actually stream those 100 games to a degree that would "beat the system", and if he did, he'd be an outliner to your average customer. The other outliner would be that guy that buys 1 game and never buys anything else and streams his 1 game he bought for the next 5 years, again I think that's more of an outliner then the norm, and if your delivery system/experience is good, people will buy things for it.
Most customers that actually buy digital and aren't terrified of the model like that peace of mind knowing they can return to said product at any time. I think an overpriced rental service for digital content is more damaging to the overall digital brand in general unless you give it some sort of attractive angle, and if they don't make it attractive at launch, it'll never catch on and will be a failure.
That being said, I don't fully understand game-streaming tech, or how much it costs to upkeep it, or if it's possible to sell games through such a service and keep it running smoothly, I'm not on Sony's Gaikai team, I just envision something a little different when I look at the service.