Multiple difficulties results in us having 4 versions of the same item, this leads to us having very boring and generic loot which also feel less rewarding to grind for. We also have insane ilvl jumps every tier which results in ''play the patch'' where only the current raid is the relevant one and the loot you are currently using is obsolete.
Also, you don't need multiple difficulties to balance the raid. Ulduar is still regarded as the best raid of all time and it only had one difficulty. It had 10 man and 25 man instead so you could technically say it had two i guess, but it still had unique items that we don't have anymore. Solution is to use the Ulduar model with some very hard hardmodes for mythic players and balance the raid for everyone. More work for blizzard, but better result for everyone, except the mythic raiders. Make loot more fun and rewarding again. There is a reason why tbc raiding is more popular than retail raiding. More fun for casuals and the loot is crazy rewarding.