You could, kind of, say Valve is the platform holder of the Steam platform.

It's their job to help make everything successful
If they don't want the burden then go third party yourself
Benefits don't come without a price

this.

and being multi-plat has extra costs.  a 3rd party like EA will be more profitable if it has 10 million customers on 1 device than 1 million customers each on 10 devices. 

Definitely true. It is however the platform holders responsibility to make 3rd party offerings viable. At least if he wants to continually get 3rd party content on his platform.

I'm talking of course for the entire platform itself, not the store. Valve have their own different philsophy on how they go about their store the same way EA, Ubisoft and CDP go about GoG, all of them handle their stores very differently to one another. Even if we count Steam as a "store" platform, 3rd parties still have to do a dandy and tight job to secure the sales, it is their job to make sure it;s a great game, not really much to do with Valve, if anything Vlave just hand you the keys to set up shop and everything just goes on from there.

I agree in a way but not fully. When it comes to making games on platforms, the main marketing is obviously the 3rd party's job, but in Nintendo's case, I think they need to emphasize the 3rd party games on their platforms as well, because that would only benefit Ninty in the long term.

Yes I agree, but it is not a third party dev's responsibility to publish games on a console if their games don't sell well. If the console audience is not interested they will go to other platforms. Also its the platform maker's responsibility to develop a good environment for other devs to grow and make games on.

Hey, hey, hey. I agree. I was just being a bit fastidious. Not even Windows can be considered the only platform. But the dominance of this OS and Steam almost make it seem like Valve and Microsoft are the platform holders. Of course, they aren't. Are they?

No worries, I wasn't trying to come off as agressive or anything =P. They could be but with 3rd parties like EA/Ubisoft/GoG and the likes of Steam I would say all of them are split up, especially since EA refuses to sell on Steam which kinda kills the whole "one leader" aspect of the PC platform, I mean if Steam is meant to be a representitive then they should really have EA games on there but they haven't since BF3 was locked down to Origin. Ubisoft games can be bought and launched on Steam but they mostly require you use the Uplay launcher in order to play those titles, that then technically becomes Ubisoft's issue to sort out. It;s basically messy when you try lumping it to one or two people =P. 

Nothing wrong with what he says.

A platform holder has to provide a platform that allows everybody to have success, but it's the game developers that have to find a way to make their games successful.