It is very hard to remove something that people were able to get for now a couple expansions and while I think the goal to make it so you cannot simply upgrade gear to get the next level of tmog makes sense to promote prestige I think a compromise could be reached by having specific items to the raid outside of the normal gear drops to mythic difficulty.
Items like the red scythe of argus or the chosen gear set from Trial of valor comes to mind. Having a couple of those could work to add some prestige rewards while letting players farm their mythic set tmog outside of the raid like they’ve been able to for the last 2 expansions. If we really want to push prestige it could also be tied to an achievement like The chosen which required mythic to be completed.
I think this could be interesting and would give even more liberty in term of what those tmogs can be as they can add a lot of flavor to them as it could even be legendary tmogs as legendaries have been something people want but controversial in term of power.
–
Side note, raising the drop rates of raids to me is still something I will always prone as if we went with the proposed change of only being able to get mythic raid tmog mostly from mythic raiding the loot droprate would also be a big issue for many.
I think going to around .3 loot per player and additional .1 tier piece of loot per player for each boss it would be a great boon to the raiding experience same as buffing possibly the amount of crests so raiding can still be used as crests catchup since you cannot infinitely farm it it makes sense that it should drop more to me.