The real answer to Monastery Mentor is winning the game. I think other people have already expressed a similar thought using different words.
Is Mentor a great card? Yes. Is it insurmountable? No. I am sympathetic to arguments that Mentor reduces deck diversity because it is such an efficient win condition, but some people take it's "awesome powers" too far.
Gush and cheap cantrips are what matters. One reason why Shops is so strong is in part because it is effective against Gush decks while the rest of the meta has to contend with Gush and Shops and then save a few slots for Dredge.
The fact of the matter is that unless a deck becomes one full third of the meta, I don't think we can even begin to claim that it is overpowered. Monastery Mentor doesn't fit.
What DOES fit (arguably) is Preordain, Gitaxian Probe and Gush. Three unrestricted nonland cards that are in more than 1/3 of the decks in the meta (not named Force, Misstep or Flusterstorm). They are a common "engine" across many blue decks, particularly Preordain and Gush.
If Gush was restricted again, my perspective is that Shops would be a little less impressive because blue decks would start playing more lands again and rollover to tax effects less frequently. Some decks would go for heavy artifact draw spells (Thoughtcast, Thirst, PO), which would just make main-deck artifact hate MORE effective against two Tier 1 matchups.
What does this even mean?