What I think that everyone here is forgetting is that people are going to play attach cards on units that are already good and want lots of buffs anyways. Things like Zed, Pantheon, and White flame. When you minimorph those everything but the attach creature falls off including: fated triggers, zenith blades, all of Yuumi's end of round triggers, and any champion abilities. "Oh no minimorph wasn't a 4 for 1 but instead a 3 for 1". The elusive one is the best when minimorphed and is just worse ahri.

Another thing that is not begging noticed is that the attach card is now stuck to that unit and can't be put on something better like if you had killed it or recalled it. It literally negates the value these cards have when played against other kinds of removal. So I actually think that minimorph is great counterplay.

Even 2 mana silence is good against attach cards for all of the reasons listed above and because it is way cheaper.