One thing about PM's question is that he's talking about a fairly short run of play. Maybe 400 hours of play. So there's still enough variance there to allow for some losing players to have winning years.
When NL first stormed the rooms, maybe 5 years ago, I decided to switch over from limit, and the first thing I did was put in a few hundred hours playing small games, because I was being cautious about this very different form of poker.
I'd estimate, from viewing the quality of play, that maybe 5% would nit-grind their way to profits, long run, and about another 5% would break-even. But for just a year, probably 7-8% would be winners, because there would be some false winners in the mix.
Also, probably, for a player who isn't good enough to make more money playing 2/5, and so doesn't step up, but is otherwise the best regular in the 1/2 games, I'd say the max expectation is ~$20/hr.
A friend who I know from 5/10, who went bust at that level, has moved to Vegas and is grinding up a new BR at 1/2, and he's been making ~$25/hr, over a large sample, but he's probably good enough to beat 2/5, so he'd be a little better than the average 1/2 grinder.
So maybe, for every couple of 1/2 games, that is, every 18-20 players, you'll find 1 guy good enough to beat 2/5, who's a little bust or just taking a day off from higher or just sat while waiting for a 2/5 and decided to just stay.
Then you'll find another guy who's a committed and winning 1/2 grinder.
You'll find a third guy who's more or less a breakeven player.
And the rest will be long run losers, but one of whom will have a winning year on luck alone.