Why can't they afford healing potions at 9th level?
They can afford healing potions, but their supply is not unlimited, particularly on long wilderness adventures when they can't just pop back to town every night. In combat at higher levels, there is not much difference between being at 1 hit point and being at ~7 hit points; but there is a world of difference between being at 1 hit point and being at 0. Why waste your supply of healing potions when you can spend one-tenth of a 1st-level spell slot instead? Better to keep the potions to supplement the party's out-of-combat healing, when those extra ~6 hit points won't just be wiped away by a stray attack.
On top of that, it also means the party does not have to forage or carry food on those long wilderness adventures. Traveling through a desert? A magic-blasted wasteland? The Abyss? Doesn't matter, your food supply is covered.
It does as much damage as Chromatic Orb but as a Reaction. That is very good. If Hellish Rebuke isn't S Tier then spells like Chromatic Orb can't be higher than C Tier. I think we're getting away from what average is at that point.
Sometimes Shield won't be enough to protect you. As far as what is on your list it's a Warlock spell so it is a bit moot. It is still quite good for a Warlock before 5th level.
I think comparing it to Shield is a mistake. I think it needs to be compared to other damage spells instead.
I completely disagree. The resource being expended is the same: One 1st-level spell slot. All 1st-level spells must compete for that same resource and therefore they must be compared to all other 1st-level spells.
There is no guarantee that
any 1st-level damage spells are top-tier, and with the possible exception of
burning hands and
thunderwave, I don't think any of them are. The damage spells are in competition, not just with other 1st-level spells, but with cantrips. Once the party reaches 5th level, I find it is rarely worth the small boost in damage to expend a limited resource when you can toss
fire bolt or
toll the dead all day long.
Chromatic orb has very little advantage over those spells, which makes it solidly C-tier in my book.
As for
hellish rebuke, I hadn't even thought about it being a specifically warlock spell, but that just makes it even worse, because of the way warlock spell slots scale. Once you hit 5th level, it is directly competing with
hypnotic pattern, fear,
counterspell, and (for fiendlocks)
fireball! Even allowing for the increase in damage from upcasting, it is mediocre to bad at that point. Very few 1st-level spells are any good for warlocks past 4th level. The ones that are useful either have noncombat utility, or can be sustained through a short rest (e.g.,
hex) so you can start a boss fight with them active and still get your full allotment of spell slots.