the marginal cost of submitting to multiple platforms is 0 so I don't think competition really does this; see also all the games available on impulse, gamersgate, desura, etc even though they sell nothing there
Even so, having a game on Steam basically means lower sales everywhere else.
If a game were on GOG, GG, etc. but not on Steam those sites would enjoy slightly better sales. But many people hold out for Steam, and once that moment comes sales elsewhere probably dip significantly.
I'm not complaining or criticizing at all - I'm happy Steam greenlit all these games (especially the adventures like Gray Matter). It's just a side observation. If I'm GOG I'm probably a little bummed that any small chance of getting some of these games before Steam has vanished.