a variant of Dominion where 2 expansions are used at once
Not a variant.
Semantics, but it still means he's not experiencing the game the same way almost every other person on the forum does, which means he has a different perspective on this particular issue from wanting to play with lots of Potion cards at once frequently. It has not been my experience at all that Dominion is improved by forcing this rule (which doesn't even have the creator's backing). Transmute is the only card I think actually benefits from it in a substantial way, and that's only in the presence of Familiar or University generally. P Stone is a weak card regardless of how many Potion cards are on the board, and has more combos outside of Alchemy than in it. Like I said, I can see the argument for Familiar boards, but just because you can make that quirk of Familiar less annoying doesn't mean it's worth doing. Most of us just like to pick 10 and go.
In full random, the metagame most people play, it's a lot more interesting to just keep it fully random, and forcing this rule lessens the amount of possible card interaction you'll see in a game, which goes against what I'm personally interested in seeing. Besides, some games have multiple Alchemy cards anyway, so those types of games get a shot in the end. In any case, this is a dumb argument that isn't going to make anyone switch sides, and the option is there to play whatever selection variant you want so everyone should just be happy.