As a population, mythic raiders have better gear faster than mythic plus players. Players who can do both have an edge over either, like you pointed out, BUT it’s a whole lot easier for a mythic raider to jump into m+ than it is for an m+ player to jump into a mythic raid.
You’re also correct in where the breakpoint is. Heroic “raiding”. Probably the largest concentration of players who fancy themselves end game wow players, full of players who were OK with second best. Then m+ came along an no ligers without a raid group pushed them to third best, and all hell broke loose.
Graph it out if you don’t believe me. I did 39 m+ last week. I think I’m over 20 this week. So let’s use me as an example, and say I average 25-30 m+ every week. I can expect 1 252 piece every week, that has a diminishing chance to be an upgrade as the season progresses. Basically, the number of non-252 slots over total slots chance, with 3 chances at a time (because naturally, I will unlock every option). By the end of the season, I’m lucky to get an upgrade every 3-4 weeks. Assuming I start on week one, there is no built in catchup for m+.
With raiders, the math works out a little backwards. In the beginning, their chance at an upgrade is essentially equal to their personal loot chance, BUT, as the raid progresses, the chance at an upgrade increases because the raid can trade the loot pool. That means that a raider joining late in the tier can roll a greater than 100% chance of getting loot on a given boss (being passes loot that the rest of the group already has). It has built in catchup, and the curve looks different. Where the m+ loot curve starts out steep then plummets, the raid curve starts slower then ramps up extremely fast.
The end result is between week ~6 and week ~15, raiders have a tier of item level supremacy on top of their special gems.
I don’t expect them to slow down mythic raiders loot, but I would like them to speed up m+ loot so that this gap doesn’t exist. Things like being able yo trade a week of bad loot. For example, if I don’t like any of the choices in my box, let me give it to someone else in my guild. Let me unlock a second pick given enough m+ runs (say 25 or 30). And I would love to be able to add dom sockets to gear or just disable them outside of the raid entirely.