Taking loot freedom away from Mythic raiders is and will break progression one way or another. Blizzard is already having difficulty tuning Mythic bosses around the top guilds and the average/bad guilds. This change will force blizzard to either make content hard for the top or hard for the average (obviously the latter is the right choice) which leads top guilds laughing at the content (think emerald nightmare). If they go the other way around... Many tears will be spilled on boards about content being too hard... Sadly over the course of Legion, we've seen this getting progressively worst. EN was a joke because tuning was bad (everything too easy), ToV and NH were mostly fine but relatively easy compared to previous expansion. ToS was a soak fest (mostly the last 2 bosses who were the only two hard ones) 1 mistake = wasted attempt (Fallen Avatar - P1 tainted matrix and P2 (rain), KJ - Armageddon). Antorus is basically the same as ToV, the first 9 bosses are all simple but the last 2 have a couple mechanics each that are unforgiving resulting in the same issue mentioned above for ToS. (Aggramar - Miss managing CCs/Knockbacks on adds, Argus... Plenty of obvious 1 shot mechanics in P1 and P2 but the real struggle was P3 pre-nerf where guilds who did not have access to a lot of immunities (mages/hunter/rogues class stacking) had basically no chance at killing the boss no matter what they did). The top guilds can manage those challenges more efficiently than the lower end guilds since personal responsibility is usually higher in those guilds but the lower end guild still had major struggles with relatively easy bosses (2/3 of the instance) due to DPS checks and player responsibility.

So once again... Taking loot distribution away from guilds might be fine when talking about easy content (say Heroic and the first 1/3 of a raid instance) but once you get past that, the "gear wall" is now random and if blizzard cannot tune fights around that, they will resort to pulling stupid mechanics out of their asses making content not really enjoyable for basically anyone doing any sort of Mythic content.