I don't buy that Mythic+ was ever intended to be a full alternative to raiding. But even if we were to assume that's its intention, it begs the question: Why on God's green earth would you choose to main a Shadow Priest if your intention was to do 5-man content? Our lackluster performance in dungeons was in no way shape or form a huge secret on the Beta.
But more importantly, suppose Blizzard were to buff our burst AE potential to be on par with other classes.
Suddenly Shadow has an overloaded kit which can do everything. We have amazing ST, amazing multi-dot and amazing AE burst. There's now no reason for any other ranged DPS caster to exist. (Cue the: "But Mages already do this," which is an entirely separate argument against class homogenization.) These are concerns which the developers need to weigh properly before arbitrarily "fixing" the issues we have. For example, if they were to shave off some of the ST potency in our kit to offset the addition of burst AE, we then lose our niche in raids. The same problem we have now still exists, but it's flipped on its head: Shadow is now more viable for Mythic+ and less so in raids.
Hence why I said earlier that I feel like the most likely course of action is no change at all. Time will tell.
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Fire is strong, no doubt. But it isn't particularly great at target swapping or dealing with multiple targets spread out. That's where multi-dot classes, especially Shadow, shine.