neurasthenya: But with this, only to an extent. The problem here being, what EA/Origin, Ubisoft/Uplay does, for the platform, besides hosting their specific games?
Because it gives the feeling of "artificiality" when it comes to the exclusivity of games (or the ludicrous bloatware in the case of a game bought on Steam requiring Uplay). It's not much of a competition as is lack of consumer choice.
Yeah, I would also prefer that games are available on several stores/services/clients so that the customer can choose where they buy a game. EA has chosen to pimp their service by making many of their games exclusive to their service, Microsoft and I guess Blizzard does the same. I guess GOG also sells Fantasy General and the SSI Goldbox games only on GOG.com, but then CDPR sells The Witcher games also elsewhere, not only GOG.
For developers/publishers it is a double-edged sword: on the other hand it is easier to maintain a game only on one service (say, Steam), but in the end it might not be good for them either that there really would be only one store/service which has a monopoly and sets their rules as they want (unless it is your own service of course, like EA = Origin). The competition between services is good also for publishers, even if it causes the extra burden to figure out in which services and in what form to release and maintain your game.
All this just makes it even more important to me that games would be DRM-free. Once I have downloaded the games, I don't have to care anymore where I originally bought them. I can install and play all my DRM-free games just fine, whether I bought them from GOG.com, DotEmu, Humble Bundle, Strategy First, directly from the developers' homepages, etc. etc. etc...
I also dislike how even hardware peripherals like VR goggles, gaming controllers etc. seem to become increasingly store-specific. For Steam games you buy a certain kind of "Steam VR goggle" and the Steam controller, what if Microsoft will next make their own VR goggles and a new advanced XBox gamepad that works only with games bought from the Windows Store, etc.? It is like these different stores and services are becoming their own platforms within the PC/Windows platform, fragmenting it further.
Multiplayer games are a trickier question (as the services/clients or even DRM can benefit multiplayer, like with anticheat technologies etc.), but then for now I've chosen to play only f2p multiplayer games. And now even not that as the latest update has really screwed up TeamFortress 2 and made it a bitch to start and join a game I want (I can't even find the servers anymore where I used to play before, and no servers seem to usually run the maps I liked to play frequently before).
As for the question which other PC gaming service could possibly overthrow Steam
in the long run: Windows Store. It will be more and more integrated to the future Windows versions and will be always be there by default, while Steam etc. clients need to be downloaded and installed separately. Over time, less and less people will possibly do that, unless there is good enough reason for it (like already having lots of games in the 3rd party service).
Plus, Microsoft can bit by bit nudge the competition out of Windows platform. Not over night, but with little moves, by making more and more new Windows features and services exclusive to games and software you buy through the Windows Store, while the 3rd party services like Steam and GOG are for the "legacy stuff", not necessarily benefiting from all the latest and greatest new Windows technology. I see it a bit similar situation if Windows 95 which certainly allowed you to still run your MS-DOS software... but it was clear to everyone that was the legacy stuff that was on its way out.
Black horse in the race is Google and Android. While Microsoft, Steam etc. are fighting for the dominance of the PC platform, Google may be getting more and more of the overall pie, especially if people start using Android devices also as PCs. You could argue that is already the case for many, like when I look at my wife who is all the time on her Android phone and tablet, and the only time she uses a PC is when doing some online banking stuff every few months that for some reason cannot be done well on the phone. I, on the other hand, are 95% of the time on my PCs, so that kinda evens it out nicely in our family. :)