There's no doubt that PSNow will eventually get PS4 games. If I had to put a timeline on it I'd say it will receive PS4 games that are 3 years old. There's no way Sony would put brand new games on there for the reason you've stated, but older games with advertising for newer games could actually HELP sell the PS4 to new consumers.
I was thinking the ssame, about it getting games some years old, but that wouldn't boost that much it appeal.
Now, getting games like CoD or Fifa there right away could be huge for the service, as it's the popular stuff people would really be interested to get on, specially the ones this service is aimed at.
And I don't know how it would affect the PS4 tbh, could go to both sides, with power. Could make a lot of people stop buying a console as well as could introduce people to games they will like and make them interested on getting a PS4 for more of brand new games of other kinds.
It really puzzles me which would actually happen, maybe it's just too risky to go for though, but then at least the last year version of that game, it would already be good enough to a good chunk and probably appeal more to owing a console to get the newer version.