Gamersgate is its' own entity and has about as much to do with Paradox today as Impulse, Direct2Drive and Steam.
Regarding multi-client/multi-source - I definitely feel that there's a paradigm shift in the industry going on right now. A few years ago (as steam was really getting dominant) people generally felt that hey "I really don't want to hassle with 15 clients - I'm sticking to one - hello Steam".
But nowadays more and more people
have to use multiple sources just because there are a lot of popular PC games that steam doesn't carry - for instance:
Minecraft
League of Legends
Starcraft 2
World of Warcraft
Soon we'll have Battlefield 3 and Diablo 3 there - those titles represent a very big chunk of the average gamer's time - right?
So yeah it's a hassle to download and maintain a bunch of different games/sources - but people are getting used to it. Besides what's to stop people from creating a 3rd party app that acts as a launcher for all these - (see pidgin
http://en.wikipedia.org/wiki/Pidgin_(software) for example)
Heck - I have minecraft, League of Legends AND Starcraft 2 on my Steam list and launch them through steam - it's really convenient because then my friends can still get hold of me while I'm playing.
So in summary - I think definitely you'll see an option to buy Paradox games directly from the "paradox store" -
www.paradoxconnect.com is the embryo of that. And that will be a great option for those who really want to support paradox and make sure a 100% of the revenue ends up in the "right hands". But in the meanwhile you'll be able to get our games from Steam, Gamersgate or even Origin (hopefully soon). Also I foresee a proper online library - meaning that if you have a game on digital service X that will mean that you'll be able to redeem that game on digital service y - see how the Humble Indie Bundle or Frozen Synapse lets you redeem the games on Steam - even though you bought it elsewhere.
Also finally - Steam is much more than a distribution outlet - as ComradeOn writes - it's a marketplace with a fantastic marketing capacity. But most importantly it also makes development a ton easier by giving developers access to tons of nifty features like autoupdate, achievements, stats, the cloud and stats to name a few.
/s