But does it become any more interesting if you just buy Estates instead of Duchies? I don't think so.
I think it would be very seldom you'd like to buy Estate for $5
Then you just buy another Rebuild. One of the tactical decisions you have to make in a Rebuild game is knowing when to switch from Rebuilds to Duchies. If Duchy is not a buy option, then you just go for Rebuild every time. Of course that's slightly weaker than if Duchy is available, but not nearly enough that Rebuild is no longer dominant on most boards. I think it's only a slight nerf, and actually makes Rebuild less interesting.
Yes, that's what I think as well. The strategy would be simple: Buy Rebuild for $5, Silver or Estate for $3, Estate for $2.
Is there a reason why the "return to supply" fix hasn't been discussed more? I think that's by far the simplest, and probably the most effective, fix yet.
I think the
simplest fixes are removing the +1 Action or increasing the price; and both have been shown to be effective for making Rebuild-BM non-dominant (the price increase to $6 probably too much so).
"Return to supply" is also a relatively simple (and elegant) fix, but I'm not sure if it really weakens Rebuild so much; have you playtested/simulated it? It could actually help Rebuild that the Duchies rarely run out in the non-mirror, similarly as with pst's proposal. (It might hurt Rebuild more if only the Provinces are returned to the supply instead of trashed, not the Duchies.)
Maybe you could just increase the Province pile size in Rebuild games, fixing Rebuild without changing the card itself?