Ya there's no doubt Steam is the most widespread and adopted service by far but if we go back in time to ~2003 people
HATED Steam and the fact Valve required you to jump through hoops and register/validate your game. Also, for the most part, if you didn't play any Valve games, you didn't need Steam.
The situation changed drastically however when other publishers started offering their games and catalogs on Steam. Some of the bigger names only recently embraced Steam (for numerous financial reasons I'm sure), like EA and Rockstar for example only made the shift as recently as 2008 I believe. In all cases its been kind of a big deal when one of them made the move and has contributed greatly to Steam's explosive growth over the last few years.
So now really Steam's greatest asset is its installed user-base, approximately 25 million, but that user-base still relies on content, the majority of which still comes from other publishers. If those publishers walked with their catalogs I don't think Steam would keep its position of market dominance on the strength of Valve's titles alone.
As you hinted at, it would certainly be a greater inconvenience for the end-user having to set-up multiple accounts for multiple services all over the place, but I think this is the reality we're moving towards as more and more of these publishers/developers want a piece of this lucrative digital distribution and subscription-based pie.