I would agree with you for the most part; currently the Bone Church (Hospitable) is only available through loot in Whirlwind Coops (which so few people play anymore). However, in the case of the Bone Church WAS available through those mystery boxes at one point (and even tradeable for a short time), so many people who currently have one (or more) on their island did NOT earn it - they bought it.
That being said, I agree that more items should be non-tradeable and only available through adventure loot (think Angel Gate, Witch Tower, Frozen Manor...). Personally, I'd take it a step further and only allow the slot who earned the most experience capable of earning said item, while also increasing the odds of getting it. That would offer incentive to actually running the adventures instead of just buying loot spots. Obviously, in the case of coops, since everyone must pull their own weight, all should be eligible for epic drops equally.

