The osX games list very much depends on you view it. Measured strictly against the size of the windows one, then yes it looks
relatively small.
Measure in absolute terms, say against the number of games you've got a chance of playing in a lifetime

, it seems quite a healthy size to me. Plenty of games on GoG/Steam.
It'll very likely be the same sort of decision with SteamOS of course. If there's stuff you can't bear to lose then of course daft to move.
The other thing is that the set of osX games on steam is currently a pretty strict superset of the set of Linux games. A lot of shared tech. You'd expect bascially everything that gets a SteamOS port, and doesn't need massive graphical power, to come over to osX too.
So in a couple of years they could easily be a very valid choice. Especially perhaps if combined with a ps4/xb1 to cover the genres where its most likely to be weak. (The same probably goes for steamOS.).