But you're sacrificing quite a bit to get there, no?
1. You're sacrificing 2 full caster levels. That means, if you multi early, your at level one spells until other casters hit level 3 spells. And that gap keeps, you're always 1 spell level behind. And you don't even get the higher slot (without the actual spells) like you would multi-ing into paladin, you're a full spell level behind.
2. You're ASI/feat is delayed by 2. Early on that's no small thing.
There is clearly decent increased versatility, but the early levels seem like they'd be a bit rough.
You sacrifice two caster levels to get
a) An at-will attack that keeps pace with a Fighter
b) The ability to easily power up that attack to be one of the most powerful attacks in the game
c) The ability to do this increasingly frequently.
The roughest level is 3. It evens out at 5 and picks up from there. The reason it isn't a big sacrifice is as a general rule (i.e. if you aren't exceedingly clever), multiclassing puts you significantly behind the standard damage curve.
Every damage-focused class except the warlock relies on class level, not character level, for its damage tiers. There's a good reason they didn't make Rogue sneak attack dice or Fighter extra attacks depend on character level. You'd get massive multiclass munchkinism...which is exactly what a Paladin, Bard, or Sorc 2-dipping Warlock is.
For a sorc, the versatility sacrifice is marginal at high levels. Early on, sure, having a 6th-level caster who can't cast Fireball is annoying. At high levels, the Sorc is down
a single spell slot. Imagine if there were a 7th level spell called this
Bodacious Blight
Casting time: 1 minute
Duration: 24 hours
Concentration: No
Your Charisma decreases by two, and you gain the ability to cast as a cantrip. As a bonus action, you can spend a single sorcery point to increase the damage by 3d6. This extra damage persists for up to one minute as long as you are concentrating.
Put whatever penalties you want on it. Proposing a 7th-level non-Concentration spell that lets you cast Blight as a cantrip would get you laughed out of the room. The basic concept is ridiculous. But that's about what a Sorlock is from 11th level on. You lose your single highest-level slot and take a small penalty in exchange for having a cantrip that is in the same ballpark as 3rd or 4th level spells (yeah, I know blight does a little more raw damage, but EB has a number of other benefits, hence the comparison). IMO, making Eldritch Blast a cantrip instead of a Warlock class feature was a huge mistake in 5e, second only two how crappy W4E monk is.