I think I answered that already in 📚 The Library on Trakt: What It Is and Where It’s Going - #13 by kcador
Basically, the data structure for physical media is not the same as watched items or lists. Also, the interaction with it on V2 made it too easy to abuse.
Those limits are intend based, not strain on our servers based. We have to keep both in mind. Not one or the other.