You aren't supposed to use Eldritch Blast when the Monster has Fireball in it's stat block.
Here is a better video that tells you what they were thinking:
Except none of them have fireball in their statblock either. The closest is the Warlock of the Fiend, who throws a 10 ft radius mini-fireball that does 3d10 fire and 2d10 necrotic. That, three scimitar attacks (which do 1d6 slash and 4d6 fire) a round, and hellish rebuke 3/day are his only attack abilities.
Compare to his Volo version: feeblemind, finger of death, flame strike, scorching ray, stinking cloud, wall of fire, burning hands, hellish rebuke EB, firebolt, shocking grasp. He has OPTIONS. He has AOE, control spells, melee and ranged attack spells, and can nullify a character for good measure. He's a good boss monster, a great BBEG and giving him a few minions makes him a terrible force to behold.
So he's gone from having a variety of abilities to shape and control the battlefield to "mini-fireball at range, melee attack if confronted, rebuke if hit". He's a three-trick pony and once you've seen his tricks, there isn't a lot else to do with him.
I've watched all of the videos on MPMM and frankly, I'm still underwhelmed. This to me is a step back to 4e's "monster does 1-3 things max" style of design, and they aren't even giving me minions to compensate. Could stat blocks have used some trimming and abilities moved from spellcasting to actions? Yes. Did they have to utterly gut caster-monster options? Not like they did.