under my theoretical model, you're still permitted to play X number of games over Y period of time without paying
I didn't really examine your model that closely, but if you only pay once you reach a certain capped number of games most casual gamers will simply stop playing when they reach that cap.
There is something quite off-putting about a model that was free yesterday, free today, then oops... you're over an arbitrary cap and now we want you to pay.
Even though it can be quite generous and you are getting a lot for nothing, it really feels like you're being tricked or swindled into paying for something that, until that point, you were used to getting for free.
I have to confess I don't really know much about the economics of online games or things of this nature, but it just doesn't seem to make sense that the only time you pay is when you pay for the sets and then that's it, for the rest of your life, even though they're running servers and sussing out bugs and whatnot. Your post implies, given the model you've laid out, that this is like any other game, you play it for a while, and then you stop, probably, and you move on to something else. I just don't think of this game that way.
Well, the issue is that whilst it isn't like buying a single-player game, it's not anything like buying an MMO either.
Have a look at the list of all the multiplayer games available on Steam.
Apart from any MMOs, how many require you to pay a monthly subscription?
What do you think would happen to the player base if Blizzard announced that you have to start paying a monthly subscription to continue playing Star Craft 2 online?
People have been willing to pay monthly subscriptions for games like WoW largely because there is an understanding that server costs are high, but also because new content is released on an ongoing basis.
Server costs for an online card game should be relatively low (especially once they sort out using local storage for game artifacts) and, like I said, new content is going to be pretty damn scarce for Dominion.
And, ultimately, fewer and fewer people are willing to pay monthly fees for even games like WoW as, for better or worse, more and more of these games move to a F2P model.
Doubling the prices for the game seems like a bad idea, but who knows, lots of things I think are bad ideas work.
The current lowest price for all the expansions is $45.
So, putting it on Steam for $20 is not doubling the price.
My point is that if they can get the game out to a larger audience via Steam, they can charge a fraction of what they currently charge and still make a lot more money.
But the game needs to work first.