Now me personally, I'm of two minds when it comes to the "streamlining" of classes. My mental picture of a Monk isn't some skinny dude in robes that would never survive the "Training from Hell" that's common for martial arts movies- I'd want Monks to have 13 Strength (or more!) as a matter of course.
So when the game says "oh, you can just use your good score for things", it bothers me a little. But when the concept of "point buy is good/dice rolling is bad" became enshrined in so many gamer's minds, it's annoying when character concept A has to invest in four different ability scores and character concept B only has to invest in three, or even two!
I mean, let's say you want to play a classic undead smashing Cleric. You need Strength to use that mace and wear that armor, Con to survive being in melee since you're a d8 hit die class that also has to routinely cast spells in melee range, and of course, Wisdom is necessary to use your magic.
That doesn't leave a lot of room to also be able to debate Theology with the party Wizard, or preach the Good Word to the flock, let alone be able to not trip over your own two feet constantly. If the game also asked you to invest in a fourth stat (say, Charisma to use d20 Turn Undead consistently), it becomes harder and harder not to be mediocre all around.
It becomes even more obnoxious when other classes are built different. A Wizard can prioritize Intelligence more than our battle Cleric can for Wisdom, and when the game decides to let said Wizard pretend to be a warrior, oh well, he gets Int to AC and extra attack, something the Cleric is never afforded.
Similarly, our Warlock friend could just focus on three stats at most and just spam Eldritch Blast all day- he has no need to enter melee! But if someone decides they want to be a Bladelock, well, sure, you get to use Charisma to hit and damage, but you still need to invest a little more of your character to just keep up with the blast spammer, who already gets Charisma to hit and gets extra attacks handed to him as he levels and might not need as much defense as he sits in the back of the party hiding in a bubble of magical darkness!
It might not be a big tax, but it is a tax, and it's not really justified just to validate the concept of the Blade Pact. In a perfect world, where the game was built with all ability scores equal, everyone wants them equally, and you weren't forced to figure out how to split your points among them, sure, 13 Str for a Bladelock would be no big deal.
But that's not the game we're given. It's lopsided and you can be taxed for wanting to play against type.
I understand there's a much larger argument about multiclassed Blade Warlocks, but I don't think that should enter the conversation about how a single classed character functions.