Originally Posted by
Relapses
Tuning is going to make 15s feel like 18s of previous expansions. 20s will feel like 22/23s. So if you haven't consistently been pushing this high (ie, stopping at 15 because that's where the rewards stopped scaled) then M+ will be a less lucrative source of "the best" gear. If you're consistently pushing then this just means that you'll scale even higher than you did in previous expansions but it will come at the cost of the dungeons themselves being more challenging.
A common criticism in the past is that because +15s were "so easy," M+ players could essentially become just as powerful as their raiding compatriots by completing weekly 15s and never actually stepping outside of their comfort zone. It remains to be seen if this change equalizes the skill threshold between raiding and M+ but I surmise M+ will remain more popular by virtue of simply being a helluva lot more accessible.