An unrestricted Windfall, sounds like it would slot far too easily into a PO deck. Turn 1, dump hand, don't have PO, but do have Windfall. Refill hand at expense of opponent, play more artifacts, and cast the PO I just drew.
Now that it is relatively clear that restricting Gush didn't solve the Blue Mentor problem (and that it really didn't solve the Shops problem, which I never expected it to), I can see the argument for un-restricting Gush.
That said, all I can see 4 Gush doing is: 1) give Oath a way to keep up with Mentor (barely... maybe), and 2) give Mentor pilots a choice between PO and Gush depending on the meta. Oh and Doomsday players, right?
I find the argument compelling that Mentor just outclasses any other win condition for blue decks and merits restriction. Regarding Shops, I also found as an Oath player that there were many times when Shops would handily kick my butt if they landed enough sphere effects plus a Tangle Wire early on. Workshop should be restricted, but I'm not sure it ever will be.
In my head, the best short-term fix for the meta would be un-restricting Chalice combined with the printing of a Red, Green or R/G hybrid mana cycle card that kills an artifact (or colorless permanent) when it cycles, thereby circumventing tax effects.
Four chalice would hinder cantrip decks and PO decks. And while they would strengthen Shops and White Eldrazi, both of those decks would have to change to fit them in. Cards that cycle to kill an artifact, or do damage to a creature, or destroy a colorless permanent, would be a wonderful addition to Vintage--so long as they aren't a blue/white spell. They would cut down on Shops/Eldrazi, but not to exclusively to the benefit of the dominant blue deck.
This is slightly off-topic, but it annoys me endlessly that blue and artifacts are so overly represented in Vintage. It shouldn't be hard to print cards in Red, Green and Black that made those colors more competitive. I am heartened by the printing of cards like Harsh Mentor and Ramunap Excavator (the latter not necessarily being good enough for Vintage). I leave out white only because White Eldrazi (or Thalia Eldrazi, really) is already competitive. In sum, I wish there was genuine color balance rather than an endless series of design mistakes for blue cards.