I can only comment on the game implementation. I haven't played tabletop since 3.5/pathfinder years ago.
In general, now that you mention all the other patrons, it seems even weirder that the warlock class universally gets and relies so heavily on the eldritch blast.
Okay, so I'm shopping around for a patron. Mr. Devil, what do I get with you? Eldritch blast? Okay, that's something. Let's see. Miss Fey, what do I get with you? Oh, eldritch blast? Hmm. Okay. Well, let's see, hello there Thing From Beyond, what do I get with you? Wait. Seriously? Eldritch blast?
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I think it's the whole class I have a problem with. Not only do I find it mechanically weak, at least in the first four levels, I also find it... lacking in flavor, especially given the potential it could have.
For instance, I think it would be nice if the different "pact types" basically amounted to a totally different type of character. One could be a hulking warrior with unnatural strength. Another a demented psionicist, while still another could become so beautiful as to charm all around.
Just a list of different things that the character could get for the pact. Like, one character might get all these extra skills while another character gets a big bonus to a particular stat, and maybe somebody else just gets an extra 40 hit points and a natural AC of 13 or something.
In combat the different pacts usually do rely on EB because the designers wanted that to be the main feature of warlock combat. Don't know why, exactly, but it's probably easier than making sub-classes that are practically their own class.
For the effects of different pacts, that's already somewhat in the 5e books. The Archfey patron, for example, grants abilities around charming and resisting charms. I don't think they can charm everyone at once, because that would be broken, but it's in that vein. And the typical combat warlock is the hexblade who specializes in a weapon and gets some nice bonuses for it. A hulking brute warlock isn't in flavor since that's not the archetype. If you want a hulking brute you need one of the martial classes, not a caster/support.
You're not wrong that warlocks are mechanically weak. They were and are in 5e as a pure class and their power was mostly as a 1-2 level multiclass dip since their features are front loaded and EB scales with character level, not class level. EB's number of attacks scales which gives you 90% of a warlock's combat capability, synergizing greatly with other Charisma classes like Paladin, Bard, and Sorcerer. Plus, you get two invocations at lvl 2 which can let you cast Mage Armor at will which synergizes with an Abjuration Wizard who can use Mage Armor to pump their Arcane Ward (basically an HP shield) between fights
There are only a few good pure warlocks and those are unimplemented pacts/subclasses. Probably because those nice features are fairly unique, mechanically, and would require a lot of coding to make work.
I love the flavor around Warlocks and I understand why you think that selling your soul to become a mechanically weak class is a bit much. But you also have to remember that a lvl 1 character is still supposed to be a special person, likely one of the notables of the local village. A person who stands out above the rest. Most people are not like that. And you can guess that a few will be so envious that they'd sell their souls to be at that level. Or so foolish as to think that they'd be able to retain their souls, or otherwise not pay for their pact. One of the best parts of being a warlock is that the DM can throw in quest hooks via the pact by having the patron desire some work from you that might be counter to what the party wants, or just open up a new side quest to entertain people for a bit and let someone RP a while.
