I'm not anti-healing. I'm just saying, all things equal, mitigation is functionally equivalent to healing.
Each of the drawbacks you bring up wrt mitigation applies also to healing.
You can't mitigate damage after the fact. You can heal after the fact. I'm sorry, there's just no way around it - not without radically changing mitigation, I guess.
The only material difference is the prescience argument, which is applicable for THP but not reactions.
What if there are 2 allies that need some help? Could you THP one and reactively mitigate the other? You could!
Not if both options took an action. And, if you needed to THP one before hand, what if it's a third that ends up needing the help.
What if 2 allies need healing? It all amounts to the same thing.
You can't see how a character who can bring allies back from 0 has an edge over one that can't?
Barring, weirdo corner cases, because there are exceptions to every rule, right? But we don't design around corner cases, we design around typical utility.
Allies getting dropped is not a corner case. Not being prescient is not a corner case. You're not-prescient 24/7.
Now here's the kicker, healing comes in only ONE form. Add numbers to current HP total. While mitigation comes in MANY forms. Both are nice and adaptable, provided you don't shoehorn mitigation to meaning only one thing. So it's dependent on delivery in a way that healing isn't. But it's free from doing only one thing (increasing HP total).
That's just another strike against mitigation. AC buff is mitigation. Doesn't help when the party's fireballed. Healing does. Save bonus is mitigation. Doesn't help when you're beaten down by an ogre. Healing Does. Resist fire is mitigation, doesn't matter when a white dragon breathes on you. Healing does. Negating an enemy's attack is mitigation. Doen't matter when an ally falls in a pit. Healing does.
I'd be delighted if the equivalency you posit were true, it would make certain things easier.
But, I'm afraid it just doesn't hold up.
But overall, sure, if you add a bunch of limitations to mitigation and ignore that those same limitations would apply to healing, then yes, healing is totally better.
Healing heals hps, it doesn't matter where the damage came from, unless your maximum hps have been reduced (there's what, one monster that does that, and it's arguably broken?). Temp hps are about as good as it gets for mitigation, because at least they don't care what kind of damage you're going to take. But whether you take damage still matters, and they can't bring you up from 0.
From there, specific sorts of mitigation all get more and more situational.
Where temps and other forms of mitigation are 'better' than healing is when you /are/ feeling a little prescient, when you know there's a fight coming, or you know it'll be with something that does a specific damage type, or whatever. Then you can use actions in advance to set up the most useful sort of mitigation for what you expect - saving actions for other uses in combat.
Which is great, but further illustrates that they're not equivalent.