Two different player bases.
LFR was for the super casual who will never join a raid, would never see the content. It was added as a way for them to complete their quest lines and at least see a very basic version of it.
M+ was created for players of all levels (though realistically it’s probably the players just above super casual all the way to the very top), as they can all join at some level and be successful (whether it’s a +2 for the casual player or a +25 for the CE player)
Mythic Dungeons you left out (ignoring the new Mega Dungeon, that’s a different beast), because they’re DEAD like you said. Heck even Heroic was dead until they added that new quest reward for doing Heroic Dungeons. As let’s be honest, you can see the dungeon on Normal (see super-casual above), and the rewards for Heroic aren’t worth it when you can just as easily get into a M+ (especially if it’s an alt and you can just ask for a +2 key and list it and get carried). Heck I rarely even see someone looking for a World Tour anymore.