But you have your other stuff like Eldritch Blast, Agonizing Blast, Hex, the Pact, contacting the Patron, etc. You cannot be a full caster and have that (plus MA / invocations) on top
Well... why not?
Yes, Eldritch Blast is the most powerful cantrip in the game. But that alone won't make it so you can't be a full caster.
Agonizing Blast? Sure, it is uniquely good with Eldritch Blast, but agonizing blast isn't as versatile as Elemental Affinity or Potent Cantrip + Empowered Evocation. I mean, plenty of full casters add +mod to their damage.
Hex got nerfed, and there is no problem with Fullcasters getting Hunter's Mark which is basically an unnerfed hex as shown by the Druid and the Bard's ability to get it.
Sure, I know that the point is the combo, because old hex was also uniquely powerful with Eldritch Blast, but since it has been nerfed, it isn't really THAT potent to deny someone full spellcasting.
Contacting the Patron? That's just a ritual spell. Everyone can do that 1/day for free, they just have to pass the save. It isn't uniquely powerful, and in fact is likely less powerful than some other divinations that fullcasters DO have access to.
The pact? Sure, these are more powerful than some of the other options, but again, the Tome is just some extra cantrips some rituals, and then an ability many other class/subclasses get.
And many of the invocations are useful... but many of them aren't that powerful either. I mean, permanent Detect Magic isn't super powerful.
Meanwhile, the ability to switch spells with only a 10 minute ritual, the ability to modify them to remove key restrictions, and then spend mere gold to make those modifications permanent? On top of subclass abilities that are basically metamagic, at-will damage on a miss, and agonizing blast for all damage spells, and the ability to crit on command with spells? None of that is enough to deny fullcaster status?
The combo of Eldritch Blast plus agonizing blast, with the nerfed hex, is quite good still at low levels. But I don't think it is enough to make it so we couldn't have MA for free starting 5th level. Because, again, a 3rd level spell that cannot be used for any other slot, and once per day, isn't as powerful as full caster progression. And if it were free, I think it would be enough, because the biggest complaint I've seen is that getting the full-caster progression in terms of single spells per level requires too much of an investment from the invocations, which can often be fun tools in the tool kit.
they are pretty much on par, but not in number of spell slots. Via MA you do have a 3rd level slot at level 5
If you rather have the features of the Wizard, play a Wizard. No one is stopping you. The Warlock is not behind in power, they just gain theirs differently (otherwise why have one in the first place...)
But why does that have to cost me an invocation slot, when before it didn't?
I'm not going to advocate for warlocks to get full-caster progression. I agree, that would be too much. But I also think that getting their limited 3/4 progression at the cost of 7 invocations is absurd. It is too restrictive.