I go ahead and say it: "made a deal to gain power" is crap fluff for a whole class. Sure, it might be how someone gains their power, but if you bargained for knowledge of magic, then you're still a wizard, you just cheated to get your degree. And if the patron actively channels power to you, then you're a cleric. Or if they transform you to be a magical being, then you're a sorcerer.
Honestly I think its good fluff because you've got immediate Story Points to draw from. You've got "Who did you get that power from?" "How was it achieved?" "Are you in contact with them?" "Are you working with them? Against them?" just right off the bat. Stuff to dig into and use throughout a campaign. Ready made allies/antagonists with big relevance to someone's sphere of power
Mechanically though the sorcerer isn't great and I'll give that, but from a thematic perspective, they're way different. Mind my dream sorcerer is less 'You cast spells' and more 'You have the raw building blocks of spells and can meddle with how they end up'. so basically Spheres of Power
Furthermore, such bargain being linked to origins of one class, makes it their thing, and puts making the bargain in the most boring part of the game: before the game even begins. Powerful beings tempting characters with power is cool, but that should be possible to happen during the game and apply to many different classes.
I on the other hand reckon the deal isn't the interesting part. What the player does with it? That's where things get interesting. D&D doesn't support you going from level 1 commoner to having class levels and its basically the same. Something Happens that makes your character have class levels
I dont see a meaningful difference between making a pact with a fiend and descending from a fiend, or making a pact with an aberration and being corrupted by an aberration.
Moreover, the Warlock flavor includes the need to study books and manipulate material components, which means thst a pact never happened, there is no short cut, and the Warlock has to do magic the hard way anyway.
My doubling down on the narrative that the body of the Warlock has been magically transformed, and thinking about the mechanical implivations of a such a transformation, clarifies what a "pact" actually means.
Would you represent famous descended-from-a-devil character Merlin, as a warlock?
Some warlock flavour could, but other's don't. You're a Charisma caster, not a book one, and if you have Hexblade or the like, well, that's not books at all.
The warlock flavour isn't being transformed though, so that'd be poorly received as a change. Sorcerer can lean into that (Especially given, y'know, Dragon Disciple being the go-to prestige class for then at release), but warlocks have always had their own seperate side of that
Incidentally to draw back to the topic, I think this is class merging side of things is another reason we don't see much push for a psion because, well, when former fully versed classes can be so quickly theoretically merged into others, doesn't speak well for any other new class coming along. Plus the psion's first showing being basically a merge of a ton of concepts in the Mystic didn't help matters