Yeah so the percentage of people getting the max vault might go down a bit but i havent really read a compelling argument why that is a problem yet. And honestly the MDI Meta argument falls a bit flat, people are running "meta" in +10s already (because they are quite stupid) so whatever level the max reward will be on, its just always a problem to play offmeta.
And yeah, if you dont enjoy M+ at all and want to competitively raid, this change sucks a bit but thats also a very tiny subset of the WoW population. For everyone else its a reason to engage in higher keys, maybe find a group or a guild which you didnt need before and feel even more rewarded for doing so. And if you dont enjoy M+ AND dont want to raid competitively, this change doesnt even affect you.
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Yeah tbf Heroic will be a bit underwhelming beyond the first few weeks. But the rest is just very exaggerated, you can upgrade M+ gear to early level Mythic bosses (with 2.5k rio which 95% of raiders will not reach in the first few weeks) so its still very worthwhile to raid Mythic for gear and not just the bosskills. Especially with the new "special" high Ilvl Gear. And yeah at the end of the patch Mythic Gear besides the end bosses will be pretty irrelevant due to the months of vault but honestly who cares. At that point you are either in extend-hell if you are a late CE guild and are happy for a few upgrades from M+ or you farm the raid in one evening for mounts or even are on break already.