That's exactly what happens though. The party retreats when their resources are expended. They may have burned through these too quickly, and in theory should be left plinking away with cantrips for the remainder of the day's 6-8 battles. But unless there's a time constraint the majority votes to fall back. If the short-resters were to push them onward knowing they're spent, they'd be risking character death or possibly TPK. There's less punishment for those who overexert themselves, but is risk to go onward when your teammates can't.
This is partly on the DM and partly on the players. If the players are used to just blowing all their resources as soon as they get them, they are going to have that problem. If they attempt to conserve resources they will get a lot further.
On the DMs end, if they are intentionally putting the party up against encounters designed to drain all of their resources, they are teaching their players to blow all their resources in the first couple of fights.
Some people like to play that way, but I'm not one of them.
But wouldn't that still be broken though? EB is broken by being so obscenely good, that the remaining 17-18 levels (the majority of the class) therefore has to be fairly weak to balance. Overall the Warlock 20 can do reliable damage, but the "fun factor" isn't there because it all comes from one singular ability.
That's what needs to be redesigned - more fun, more options, less constant EB spam and more incentive to go further than three levels.
I wouldn't say it's broken. Fighter gets Action Surge at level 2, which remains very powerful and useful for the rest of their career.
Maybe I'm just valuing Invocations higher than others. Let's see what our basic options are for those. (I'm going to ignore Bladelock because I do think it is broken.)
Free spells: 11 Invocations grant the ability to cast a spell (none of which are from the warlock's list) at-will, without using a spell slot. The spells range from 1st to 4th level.
Blast enhancements: 3 Invocations improve
eldritch blast
Spell access:
7 Invocations allow you to cast a spell that isn't on the warlock's list 1/day, using a spell slot.
Boon enhancements:
5 Invocations are designed to improve on what you get with your pact boon.
Sense enhancements:
4 Invocations grant you new sensory (or extrasensory) capabilities
Miscellaneous:
1 Invocation grants you two skills, and 1 grants you a limited ability to become invisible
Tomelock and Chainlock both have a good Boon enhancement that they should probably be taking, and they should also be taking Agonizing Blast. So let's assume they take those, which leaves you with 6 discretionary Invocations to select from during your advancement.
The spell access invocations are unappealing, so let's just get that out of the way to start. While I understand why they are designed as they are, it means that you have to
really want a particular spell to take one of these. As a player
I probably would take one of those, but I'm not going to assume anyone else will for these purposes.
The sense enhancements and miscellaneous invocations give you new, non-combat at will abilities. The kinds of stuff other casters would use spells for. Except you can use yours all day. I'd think most players can find at least one thing they like here, if not more.
The discretionary blast enhancements are purely up to the player. You don't actually need them. But they are pretty cool if you want them. So excellent design here.
Which brings us to my favorite: free spells. You can choose from
mage armor,
levitate,
speak with animals,
detect magic,
false life (1st level version),
alter self,
disguise self,
silent image,
jump,
arcane eye, and
speak with dead. This right here is my favorite part.
So after taking your boon enhancement and Agonizing Blast, you have 6 Invocation left to spend on free spells, blast enhancement, interesting sensory upgrades, and useful miscellaneous abilities.
That's pretty exciting to me. Would I like to get 12 rather 6? Of course. But the same can be said for any class's abilities.
I think part of the difficulty with "getting" warlock is that its features are spread out into 5 different categories.
Eldritch blast, spells, Pact, Boon, and invocations. Most classes have their features spread out into about 2 or 3 categories.
People also see spellcasting and think that should be the basis of their flexibility. But that's not really how warlock is designed. You are splitting your "spellcasting" flexibility between spells and invocations. You can't look at spellcasting in isolation from invocations.
Now, I'm not saying warlock is the best designed class out there. It's its own thing, and it can be unwieldy because of it. But I don't think the class is broken in the sense of being underpowered.