Sure. Not in every case. Just most cases.
Like the CR 10 Aboleth and it's 12 average damage? Or CR 5 Cambion with it's 11 average damage? Or CR 4 Chuul with it's 11 average damage? Or the CR 6 cyclops which as a giant is supposed to dish out lots of damage, but barely exceeds 18 with it's 19 average damage? Or CR 5 Barlgura with its 11 average damage? Or the CR 9 glabrezy with its 16 average damage?
18 is sufficient to give someone a decent chance of staying up against many, many things of decently high CR. And if you're that level, your heals will be better.
The Aboleth which can hit three times for 12 damage, curse you with a disease to be unable to recover hp unless underwater, which cannot be removed except by a 6th level spell? And of course, we all know Aboleths are physical threats, not like they can enslave the Fighter to carve you up for far more damage.
The Cambion, who can attack twice for 11 damage and can charm people as well.
The Chuul who can attack twice for 11 damage, and sets up for a 1 minute paralysis
The Cyclops who attacks twice for 19 damage or once for 28 damage. (BTW, hill giants do two attacks of 18)
The Barlgura who attacks once for 9 and twice for 11 damage.
The Glabrezu who can attack twice for 16 and twice for 7 (or cast a spell)
So, yes, you can cast a healing spell as your entire turn to handle between 50% to 33% of a monsters turn. But hey, that might be enough if these monsters only hit once instead of twice. Yes, Cure Wounds can be enough to stop a single attack, but when you are dealing with multi-attack and cure wounds takes an entire action, then you can't look at just the value of a single attack. I mean, you are trying to sell me that it is useful to heal from 10 to 18 hp agaisnt a monster that can casually deal 40 damage a turn. But in practice that was a waste of my action.
No. We're talking hits, not specials with saves, and dragons to boot. The claw averages 15. The tail 17. The bite barely over 18 and 19 damage.
Of course you can cherry pick something that does lots of damage with a special attack. Most things do not. Besides, if you can cherry pick a CR 10 red dragon, I can cherry pick resistance spells and/or tieflings PCs that are resistant to it and make their saves, only taking 14 damage from that breath.
No, we are talking healing. And I'm not cherry picking. I talked earlier about how cure wounds can't counter burning hands.
Also, again, stop being disingenuous with the monster's abilities. Cure wounds takes your entire action. That dragon doesn't do 15 damage (it also doesn't have a tail attack at all). Instead it does three attacks for 13+13+20 or 46 damage. Sure, you could cast a 2nd level cure wounds as a life cleric, healing 2d8+4+4 for 17 hp, and stop one of those attacks. But you only have three 2nd level spells.
Meaning that if you did that every round, you will prevent a single round of damage from the dragon, who isn't using their iconic ability. You can heal 51 damage, and in that time the dragon will dish out 138 damage. And even if you increase that to using all of your 3rd level spells, instead of your second, you are only healing 67.5 which is just barely half of the damage being dished out. For three beefy spell slots, from the "best" healer in the entire game.
Also, we are not talking about Tieflings or Genasi, because we aren't talking about resistance. If I need to have resistance to the damage for the healer to be able to keep up, then the healer can't keep up. And while I may be in an all tiefling party that all make their saves, I could also be in an all-dwarf party that fails their saves. Or an all half-dryad homebrew party that takes double damage because of vulnerability. Once we go that route, we are going to get uselessly bogged in the minutia that frankly does not matter.
