I have not played WoW, I played Mabinogi which every piece of content had a "beginner", "easy", "hard", "extreme", etc tier that the dungeon actually changed for, but ultimately the monsters were just buffed up, and you could still faceroll it by bruteforce throwing bodies at it, or spamming hundreds of pots.
Great, so you actually enjoy a mechanic that another MMORPG has. That doesn't mean it would work here, where all the dungeons and monsters are balanced against the jobs.
Yet the actual description for Mythic+ is literately that. I know there is more to it, but you're ignoring the elephant in the room that the FFXIV content doesn't scale in that direction.
Because the raiders are the ones that present all the bad ideas that would create nothing but DPS sponges if no thought is put into it.
We have a Tank-Healer-DPS setup, Increasing the HP of the mobs only increases the time it takes to kill the mobs, it adds no additional mechanics. And a lot of that comes from the trash mobs in content lacking any kind of defensive actions. There are never "healer" mobs, there are rarely tank mobs in dungeon content, only in guildhests and solo content are healers and tanks ever present.
One of the key things missing in existing content is not having the mobs play by the same trifecta. If the content was to be "made challenging" the entire RPS scheme needs to be fit. eg Party tank attacks belligerent DPS, Party healer heals party tank/party DPS, party DPS attack belligerent healer first and ignore belligerent tank, or split the belligerant's tank attention to make them stop defending their healer. These things exist in the solo content when the belligerents are other player-character npc types. Monsters don't do this at all, and some bosses have add phases that are closer to throwing DPS, not healers into the battle.
And I imagine the reason for this is that SE doesn't see any value in making the trash mobs an endurance test either. They're road cones, nothing more.
This is a complicated matter, suffice it to say, basically I suspect there is a level table the server follows when it spawn mobs, and thus all mobs of X level have the same HP in the same way that players with no gear would. Player-type mobs have gear, monsters that aren't bosses do not, hence the consistent HP/MP.
IMO, all bosses should have an enrage timer, even MSQ. Obviously the reason there aren't on MSQ is because the content needs to be clearable by players who haven't played any of the content (jump pots) to understand the mechanics, so they're more forgiving than they need to be. As I've said in the past, existing content's difficulty curve is basically a low bar to clear, and then suddenly the end content spikes up because the gear creep no longer allows it to be a faceroll.
Like existing content has 90 minutes to do content that can be cleared in 10 minutes if done perfectly. There is no reason an enrage timer should be hit except to say "you're doing this horribly wrong"