I tend to agree with Rich Shay on this one. If vintage was a magic online only format and isn't the format that it is and was more in line of something like Modern or Standard, Shops should be restricted. It's clearly above and beyond better than a lot of restricted cards. It's not even close.
However, vintage is a very unique format. It's not a GP format. It's not a pro tour format. It's a fringe format that some of us are extremely passionate about. Typically, I would say we can't consider player's emotional attachments into b/r considerations. However, something like Mishra's Workshop, in my opinion, is different. You may feel different. You may look at it a lens without emotional decisions. I can't blame players for that one. However, I cannot to be honest.
It's the format where people spend 10,000+ dollars on a deck. I have a 2 Card Monte deck- probably the most expensive deck in the format. I own all of the P9 and 4 Mishra's Workshop. I currently own zero dual lands, no Force of Wills, and 1 single fetch land (wooded foothills). It would be extremely frustrating to have 2,000 dollars of that deck be invalidated. The deck could still be playable sure and, yes, I could sell those cards and get blue duals and be back in a blue deck (not the worse thing in the world). However, what about the players that don't own blue duals or the blue only p9 cards? It may not be feasible for them.
If it was restricted, I'm not sure I would continue playing paper vintage. There's a great chance I would, I'm just honestly not sure.