If you are making a completionist build, unless you TR life is only 1 week long and you constantly at it.. you need to make long term plans. Many players don't have a whole lot of faith in the developers about their concerns so that is why they are especially loud on the issue.
I kinda wonder how much you have TRed and if you have even maxed all your destinies on any of your toons once.
I also wonder if you ever spent any Turbine points on tomes of fate or keys to unlock faster certain destinies. If you had you would think that such would be a nice investment because you believed that you unlocked it forever. Now with this proposal you would feel inclined for a refund on your turbine points.
So people are upset on this proposal and expressing it. Some of them have legitimate concerns. Some people don't really care cause it doesn't affect them personally one bit of difference cause they haven't gotten that far in game. Either way it is too early to tell if it worth quitting over as they may feel inclined to not even touch heroic TRing. We can however respect that the majority of the experienced player base is not particularly happy with the idea of being forced to lose epic destinies and any investments they made when it was told to them that EDs would not be touched by heroic TRing when EDs came out in the first place.