Personally, I try to buy PSN games when I get the chance because I simply love the idea of most of them being a hop on/ hop off type deal, if that makes sense. For example, PAYDAY: The Heist (although not worth a full $20 ($15 is reasonable but too pricey)), is the type of game where I can just go to any mission I want play it for 20 minutes or so before I go out and turn it off whereas starting a whole new single player in Mafia II where you're forced to complete chapters at a time.
Great example, PixelJunk Shooter, a game where the only sort of plot is to save some survivors and I played it for 10 minutes or so before I went to bed and I had a great time. Just because I haven't played it for a month or two, I'll end up losing the plot and having to restart to get a grasp of it. Instead, I can hop on it before I do something for 10 minutes and just have fun instead of, kill this guy because he killed your buddy's friend's dog or something like that.
On the issue about the plats not being on PSN titles as much, I do believe they have an affect or simply just trophies on it aswell because I've personally skipped out on some supposedly great games like Sideway simply because I didn't feel like earning 3 trophies after completion and having to collect collectibles for 100% completion. Along with that, I think PSOne classics if you want to add them aren't bought as much because of trophy updates and HD collections coming so it questions, why play them?
I do want to add my personal opinion for this though, it seems as if when a PSN game doesn't get rave reviews or doesn't have a big named company attached to it, it simply doesn't get bought at all. Games like Rocketbird: Hardboiled Chicken is one of the top PSN games I want to buy but I haven't seen anyone playing it for I don't know what reason but they'll just end up buying the next Telltale game or whatever PixelJunk throws out next. I have to look more at the PSN games to back up my conversation but hopefully you get what I'm saying.
And for the final one of has the quality fallen? I don't believe so due to the fact games like Journey have proven to give you basically a journey that doesn't cost $59.99 to play. Games like Battlefield 1943 gives you that great DICE multiplayer so much of us have come to love but without the $59.99 price tag including a pointless single-player, instead it gives us what we all love which is the multiplayer for 1/4 of the price (although there's only 3 maps ). There are great PSN games in the store IMO that puts most $59.99 retail games to shame by providing a 6-12 experience, extra game modes, and sometimes a platinum or fun trophies for a quarter of the price of a normal retail game.