So I opted to see what the difference is between the two warlocks by the numbers.
I quickly made a 7th level warlock. Fiend patron, chain pact. PHB only. Let's look.
2014:
Invocations: Devil's Sight, Armor of Shadows, Agonizing Blast, Voice of the Chain Master
Cantrips: Eldritch Blast, Mage Hand, Minor Illusion
Spells Known: Arms of Hadar, Hex, Burning Hands, Misty Step, Scorching Ray, Fly, Counterspell, Banishment
Spell slots: 2, level: 4th
It's hard to adjudicate how many rests a warlock can get, but let's assume they get two per adventuring day. So with that, they get 6 spells per day, but no more than 2 before resting.
2024:
Invocations: Devil's Sight, Armor of Shadows, Agonizing Blast, Mystic Arcanum (Banishment)
Cantrips: Eldritch Blast, Mage Hand, Minor Illusion, Fire Bolt
Spells Prepared: Pact Familiar, Arms of Hadar, Hex, Burning Hands*, Command*, False Life, Misty Step, Scorching Ray*, Suggestion*, Blindness/Deafness, Invisibility, Web, Levitate
Spell Slots:
1st: 4
2nd: 3
Plus one free casting of a pact spell (*) and one use of Banishment per day.
Clearly, the 24 warlock has a much wider selection of spells, thanks to access to the full arcane list and all the bonus spells. The free cantrip and hex further adds to it. The cost obviously is having at best 4 2nd level spells and 1 4th level vs 6 4th level. You simply can't compete with spamming banishment six times. Of course, that came with the cost of not doing much else: each use of misty spell cost you a banishment. In fact, nothing quite came close to it in terms of power in your kit, though maybe counterspelling an opponent's 4th level spell might have (not that monsters cast spells anymore, but I digress).
You might want to argue I didn't pick the best choices per level. I wasn't assuming optimization, but an attempt to make a well rounded caster.
FYI: A 7th level sorcerer who converted all their spell slots to sorcery points, and then used them to create 4th level spell slots could cast banishment... 4 times max per day. In case you wonder how good two short rests are.
