I would also argue it's actually good for mythic players too once they get over their feelings.
Yes, i understand it's annoying seeing a random with a 955 Grand Design Heroic TF when you only had 940 Mythic, but on average mythic raider will still have average ilvl that is about one tier above heroic raiders with the same number of boss kills. We do observe this in Legion even with other factors such as m+ and legendaries.
I was pretty against unlimited TF before but honestly having a chance to be rewarded is better than absolutely zero chance. The other side argument of "feeling compelled to do older content for chances" is mostly just philosophical too, unless people that complained were chasing high TF CoF/Whispers were actually doing it on all 4 difficulties then.