HP bloat is a fault of 5E's shit math. Since defense caps at 20, and you reach that cap at level ~4, they had to give powerful monsters some other way of not dying instantly. Problem with that idea is that damage for martials caps around level 6 (when they get to attack twice) so
1: Fights just become a slog since its who can drain the other's pool of massive HP first
2: Since offense can't scale much either, there's little functional reason you can't fight anything in the system by level 8 or so since all they can have is more HP (but not more offense) and a few immunities (which only matter if you focus on that thing) so it's just a question of how much of a slog you're willing to deal with.
it sure does sound like fights would be boring if you're exclusively pitting martial PCs against martial enemies. but even then:
1. the ways powerful monsters don't die instantly are: a) multi-attack; b) spell-like abilities and c) legendary actions and resistances. HP bloat wouldn't even be sufficient for extending their lifespan if not for the previously mentioned items.
2. you're assuming no magic items, which is, obviously, not reality. for defense, non-magical plate and shield give you 20 AC, either one of those can be +n. then you can add +n rings and amulets, then if you can cast shield that's +5, then if you can take the defensive fighting style that's a +1, etc. for offense, plenty of magical weapons add extra dice of damage and some items increase spell damage.
3. Polearm Master, Two Weapon Fighting, Crossbow Expert all give you more than 2 attacks per turn. spells like Haste also give you more attacks, and others like Holy Weapon add extra dice of damage. Hex, Hunter's Mark, Smite and Cantrips all scale with level. hell, Eldritch Blast lets you get 3 attacks without any feats. class features like Sneak Attack and Rage scale with level. Great Weapon Master and Sharpshooter allow you to trade to-hit bonus (which scales) for damage.
point is, fights can be lethal for both sides even at high levels. if can really kill anything at level 8, your DM must be terrible at tactics.
Yep. Also, a mage in lv 5 can deal merely 8d6 damage with fireball. At lv 11, 10d6 with freezing sphere. 6 levels obtained and merely +2d6 damage. While a CR 11 mob has multiple times the hp of a CR 5 mob.
if you're using damaging spells as a mage, you're doing it wrong. Wizards in 5E almost always have ways of ending or trivializing encounters with a flick of the wrist. the great thing about is that they have a different win button for each stage of their adventuring career. at low levels, it's Sleep, maybe Hypnotic Pattern and sometimes Web if you want to let your martials play a little. at mid levels, it's Polymorph, Hold Monster and Banish. at high levels, well, at that point you can just hang out in your pocket dimension and let your thralls/golems/undead do the dirty work.
