Many collectible card games limit the cards that are currently "legal for play" to something smaller than "every card that has ever existed". While the cynics might say this is merely a money-making scheme, it allows the game developers to tweak power-levels, adjust game balance and introduce card mechanics in a way that would be more difficult without hitting the reset button.
Have you considered taking this approach with Dominion (or at least Dominion Online)?
There's casual play and then there's tournament play.
For casual play, you can already do whatever you want. For Magic you might want to be able to say "this is a Standard deck" or whatever so that I'm not playing my deck with Moxes against your Spider theme deck. For Dominion there's no such concern.
For tournament play, I recommend not playing with certain cards (especially the slowest cards), or being careful with the set of 10 if you want to include certain cards. If we ran lots of tournaments I would have a list of cards to not play with. We don't. If you are running a tournament, I don't mind you doing whatever.
For Dominion online, I advocate players getting to have a 5-card ban list, and letting you play rated games that will never have your 5 cards (or your opponent's).
All of the Dominion expansions have to deal with the power levels of all previously published expansions; for all I know, someone gets Dominion and then Prosperity and then Nocturne, and that's what they've got. The Nocturne cards don't want to suck there, or be too dominating there. There's only so much you can do but every time I try to do it.
I have not really considered letting expansions go out of print (and I doubt RGG would be interested, yes it happened due to the switch to second editions). It was proposed by someone once and well. It turns out that at a supermarket, people spend more if they go clockwise through the store, or is it counterclockwise, I forget. It's one or the other though, you can look it up if you're interested. So you can build your supermarket to take advantage of this (via where the entrance is), or to thwart yourself, or try to ignore it (and possibly thwart yourself that way). Jay and I are just not people seeking to maximize profits via whatever it takes; we're okay with people walking through our supermarket the wrong way. And uh I don't need to sell you one expansion instead of another; I'm happy with people buying the old products.