As was already mentioned if the install base of PS+ grows the publisher potentially loses money by releasing on PS+ versus retail and digital sales. If 1 Million download your game and Sony paid you a couple hundred thousand you lost big time, if 40 to 50k people download your game its not as big a deal.
Another potential issue is buyer reluctance - we already are seeing lots of discussion from gamers stating they expect to such and such title on PS+ in a few months so they don't intend to buy it.
Sony is going to need to identify new ways to attract publishers to PS+; money ultimately is what will do it but Sony doesn't want to be spending loads of cash each month to subsidize gamers entertainment. A couple alternatives are Sony shares the revenue generated from monthly subscriptions or work with publishers to provide a F2P alternatives that draws players into the game and then has them spending money.
One way I can see this working is if Sony fronts the publisher some cash and the publisher uses this to rework the game to facilitate F2P mechanics and then releases the title on PS+. As we get further into this generation this might be less necessary because developers will start to design more and more titles with F2P alternatives in parallel with the traditional version of the game.
Another alternative would be releasing early chapters of the game, say chapters 1 thru 4 of a 16 chapter title with the rest locked out unless you pay 10 dollars for 5-8, 9-11 & 12-16 respectively. The impact would be similar to a demo but without the time restriction. Essentially a version of episodic content delivery.
Regardless smarter people than me will have to work out alternatives and this issue runs deeper than PS+, there is a big gap between hits and everything else that developers create and their investors pay for. The reality is that at some point its too risky due to cost for developers to take chances. Gamers like to be entertained but they aren't very excited about paying 60 dollars every time they play a new game. They'll do it occasionally primarily with AAA big budget titles that are pretty derivative year after year but the market is much bigger than that.
The movie and TV industry give some clues about what can be done such as a combination of ads, drip purchasing (F2P) - this is essentially what GOT or Walking Dead do each year, you can go online and watch the first episode for free and then your cut off. Hopefully for them you're hooked and will sign up for the cable package that includes the show at that point. Regardless how ever it evolves some new distribution model is bound to become increasingly important bc retail, digital,used games and F2P is not providing enough alternatives, especially for small studios with great ideas but little to know capital trying to get a noticed.