That's exactly how it works though. If you raid heroic, and do like one M+ a week, you're absolutely going to be stronger than someone who grinds world quests/maw/torghast/etc for 8 hours a day, but who doesn't raid heroic.
The problem is that there are heroic/mythic raiders who also grind hours every day, and those people will absolutely be stronger than you, but that's to be expected, because they're playing more.
The TL;DR:
Harder difficulties == Large boost in player power
Playing more hours == Small boost in player power