That’s not an m+ problem, that’s a raiding problem. M+ being good for gearing isn’t what takes people away from raiding, raiding does that all by itself. If mythic raiding were the only place to get anywhere near max ilvl I would quit the game before going through the trouble of finding a mythic guild and dedicating hours long chunks of time multiple days a week…and think many would as well.
If you separated each game mode allowing only-raiders and only-m+'ers able get the same gear of the same quality doing only their preferred game mode…you would notice a big dip in raiding participation while m+ participation wouldn’t be noticeably different.