Once the junk is gone, your engine should be humming. Whether you got to that state with Junk Dealer or Hospital, it doesn't matter -- the card has done its job. You don't need to trash one of those Swindlers because you don't need to play your trasher anymore. Your junk is gone! That was the whole point!
Let me try a different approach here. Consider these two prototypical cards:
Cantrip Trasher
+1 Card
+1 Action
Trash a card from your hand.
Cantrip Coin
+1 Card
+1 Action
+$1
Which of these two do you think is stronger? How expensive should each card be?
Well the engine is only humming if you have more "splitters" other than hospital - OR.....more cantrip coins. Therefore I consider cantrip coin a more useful card. I would price it at $4 and cantrip trasher at $3.
Cantrip Coin is correctly priced at $4. However, Cantrip Trasher pretty much cannot cost less than $4. The cantrip trasher is
by far stronger than the cantrip coin. At $4, the cantrip trasher is arguably stronger than Sea Hag, the highest rated $4 card. If you put it at $3, I would probably open with two of them more than 90% of the time that I could. At $4, I would open with it just as often. It's actually arguable that the Cantrip Trasher is
too strong for $4, but it can't cost $5 because Upgrade and Junk Dealer exist.
You are correct that "cantrip coin is never a card you don't want to play" (ignoring edge cases...), but that is not a great indication of strength. Pearl Diver is also a card that is always fine to play, but it is really weak. The
impact of cantrip trasher is far more powerful than cantrip coin. Trashing is that good!
I was hoping you would at least acknowledge that cantrip trasher needs to be a $4 card at least. The next comparison would have been to buff both cards to make them Village Trasher and Village Coin, i.e. Hospital and Bazaar. I guess that approach has failed.
It really sounds like you are unfamiliar with engines in Dominion. You keep saying that the strategy fails once the junk is gone, but it is exactly the opposite. That moment is not the end, it is the beginning*. Once the junk is gone, your deck is free to do great things.
*Though actually, the beginning is a little earlier because you should already be building and accelerating before the junk is completely gone.