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.
Good points, and the prospect of something like "rent to own" is enticing. Perhaps a hybrid PS Plus model would be the solution to offset server costs? Namely, for monthly fee, of say between $2 and $5 a month, you get indefinite access to any digital games you've purchased through PS Now and any PS3 games you've bought digitally in the past. Or better still, just go with the Netflix model and levy a monthly fee during which players can play any game without limit, maybe with a little extra for games less than a year old or something.
At least we should remind ourselves that this is a beta and Sony will be gauging the response to their pricing and general interest in the service. Their track record in value for the gamer has been excellent considering the PS Plus legacy, so I hope this continues.