I'd say it'd make sense if crystal players above, say, 1000-2000 points wouldn't get matched with diamonds anymore but that's more for the crystal players sake if anything. Pulling people who're putting in time trying to rank up on the crystal ladder into diamond games shouldn't happen past a certain point IMO.

Having said that, I don't really understand the complaint of getting matched against crystal from a diamond players perspective. You've had to deal with people one rank higher than you throughout your climb so why is it suddenly unfair at the last moment? If you're trying to climb to crystal then surely you should be able to somewhat hold your own against them instead of just stomping on plats. Fun and frustration are subjective but calling it unfair is a bit of a stretch.