As a lowly heroic raider and 1100+ raiderio, I dont think mythic 10 is harder than heroic raiding. This tier, we were farming M10s the first week it was open. It took us a month+ to finish the raid. Mythic 15+ is probably as difficult as mythic raiding, with M20+ being as hard as Jania, not that it matters, since you dont get higher gear. Hence him saying, M+ invalidates everything but mythic raiding.
Without tier sets, heroic raids feel like they are hardly worth the time to farm. Kill all the bosses once, quit till next tier (other than 1/3 of the raid team wont come back after a 2 month break). You can gear up faster and easier in M+. I guess that is intentional, to incentivise the group to move on to mythic raids. I would rather just quit raiding, than deal with mythic raiding guild drama and 100 wipes on bosses you already saw / hated in heroic ( fuck you Mistress Sassz'ine).