I think the bigger issue isn't that your bis are in M+ but 3 other things:
- incredibly bad itemizatiom and very limited loot options in raid + no way to ensure you actually get any loot for several raids
- raids are scaled assuming you do M+ and craft your 418 items
- some bosses on x difficulty are harder than x+1 difficulty; assuming you have the required numbers to do mythic or pugging mythic to get past hc razageth for example. Otherwise you just tell a hc only raider he needs to be better than a Mythic raider (because gear won't help as much).
This overall makes it for an unpleasant experience if you only wish to raid with your guild. Raids should be scaled assuming you only do the raid. Anything on top of that should make your life easier in raid. And I say this as someone who does M+ religiously.
As a note, if you don't do M+ and only do raid, half your raid is probably carrying your ass while doing M+ etc and feeling very bitter about it.