For the sake of clarity, you are against being able to open chests with Heroes other than the one you won the chest with, right? However, it is hard to understand your reasoning behind why this is a bad system. Because people don’t have to play a specific Hero to get items for that Hero? I’m not sure how that is a negative, though. As others have rightly pointed out, this drives flexibility in terms of Heroes played. Having to start collecting power all over again with each of the 5 different heroes would undoubtedly be fuel for frustration. Additionally, the current system allows us, the players, to maximise our chances at gaining the rarest items by opening chests with high level Heroes (which would mainly apply for new or intermediate players). Why would you want to remove one of the (few) things in the game that gives power to the players?
Removing chests after a small period of time is like something out of a Korean MMO. If anything, the loot system should be criticised (which it is) for not giving even more power to the players by, for example, easing the process of gaining desired properties and traits. The focus of the game is not loot, it is the challenge and the mental rewards that come from being challenged.
Hopefully you can understand why everyone is reacting negatively.