I think there's a fundamental disagreement that is based on metrics, one of which I think is defensible and the other, not so much.
There are sets of folks who think Gush Decks are the problem. Agreed, because the number of decks that run Gush is super high.
There are sets of folks who think The Mentor Deck is the problem. Agreed, without the Mentor deck, the current gush-based meta is miniscule.
So which is the best way to look at this?
Personally, I don't think lumping "Gush Decks" into one category is a useful metric when talking about restriction. They share characteristics, surely, but some are clearly better than others, and they are not equal. Some non-trivial percentage of the total number of Gush-based decks are decks that are causing the current problem, ignoring the fact that those causing the problem have the card Monastery Mentor as the crux is irresponsible. When people say "Gush decks" within the context of this entire discussion, it is about Mentor decks. It is revolving around Mentor decks. It is the basis for analyzing what will happen to Mentor decks. If all Gush-based decks are a serious problem in the current metagame, those wanting Gush restricted will probably have to show why Pyromancer Gush, Nahiri, and Gush/Bond are problematic in the current meta, or that those decks will be a problem in a Mentor-less meta, or were a problem before Mentor was printed (sans Nahiri).
That's where stuff gets very tricky.
The argument that the currently un-problematic Gush-based decks are still somehow destructive to the metagame is one not based off of data, or the current/past metagame. I agree, Gush being in the format is stifling to other archetypes, but calling for a restriction based off of a personal desire to see an idealized state of vintage is not defensible, especially if we're calling for official DCI intervention. Restricting Gush may be "good for the format", but it doesn't solve the immediate problem which is the Mentor deck. If we want Gush restricted because of some generalized feeling about how vintage should be, then I don't think it's reasonable. Remember (not so long ago, and currently, forever) people were opposed to the card Mishra's Workshop because of a notion that people should be able to play spells because they're playing Magic: The Gathering, and Workshop "enables" Sphere and Thorn and Tangle Wire? Same thing.
On the flipside, if we view the "Mentor Deck" itself as the problem, we can easily see that the mechanics of the central card benefit greatly from more cards in the deck than just Gush. We see Probe, Preordain, Jace(s), Dack, and a number of other restricted spells that generate the game state none of us enjoy being under. Gush isn't even a fundamentally required part of the Mentor deck. Restricting Gush doesn't hurt this engine, there are ample replacements for Gush just waiting to be utilized. The problem of "Gush Decks" being the best decks would be solved immediately if 100% of the Mentor decks switched overnight to U/R Delver/Pyro-Gush. Since that's not going to happen, I'd imagine some of that to be spread around nicely. The reason nobody's talking about Pyromancer in terms of Gush's restriction is those decks are far less menacing and it doesn't fit a narrative that Gush is the sole reason Mentor is king.
So... it seems like if you've always wanted Gush to be restricted, "Gush Decks" have always been the problem, so Gush is the problem. That's just not good enough. I think a lot of people are using Mentor's dominance as a means to leverage their distaste for Gush without actually thinking about what would happen to the Mentor decks and their standing if Gush were restricted (very little).
I'm not even a Gush player, I'd love to play weird brews and decks enabled by old cards that people don't use anymore, I like "breaking the game" and finding fun, unintended interactions between cards. That's what draws me to Vintage (and Magic in general). I rarely can do this. Seems at odds with my defense of Gush? I just think people need to be reasonable in their assessment of what's actually wrong with the format right now and what's the most effective way to fix it with the least amount of steps possible. If something Gush-based emerges after Mentor to be annoying, unfun, and choking the meta, then look at Gush.
And think about it... If Gush is gone, you will have to rework the metrics of "Gush Decks" to "Mentor Decks" anyway since the deck will still be regularly making top 8s.
So I guess the question is, is it "Gush Decks" or is it "The Monastery Mentor Deck" that's causing problems?