Good points.I feel the same about burnout, but it's really the player's fault as much as the dev's.
Making AP endlessly available was bad because a fair amount of players felt compelled to grind it until their eyes bleed, which results in burnout (from doing MoS 500 times or whatever).
Others felt they were behind and couldn't compete because they only have 35 as opposed tot he guy with 54 wep traits, so in dps terms they were naturally down like 10%ish (or whatever it math'd to), which is a lot in the grand scheme of things (like equal skill/gear making a difference of 50kish or more).
Legion made everything repeatable, but to a fault, so people repeat the living hell out of it and get bored/burned out.
In past xpacs, burnout happened, but at a slower pace because your "effective" play was limited per day/week (mythic dungeon lockout in WoD, heroic dungeon lockouts prior, raid weekly lockouts, shared lockouts between difficulties like back in the day, things like that).
Sure, you could log in and do something outside the normal raid routine, but odds are it won't be something that makes you more powerful.
Now, effective play is basically every minute in game (as in you can gain something, whether it be gear or AP, at all times), so this ramped up play time and thus, burnout.
It sucks, but that's what happens when you play the crap out of a game; you get tired of it.
Also a good point.A lot of players don't feel wasting their time in mythic encounter anymore. With Blizzard Warfoged/titanforged system, someone who does his Heroic run each week could be equally or even better geared than someone whos running Mythic raid. ( and it's even worse if we consider trinkets. )