warlock patrons CAN'T be major NPCs because they'll only ever exist for warlock characters.
It's amazing how some D&D players believe every table plays using their house rules! This is total nonsense. The patron can be as big an NPC are the DM wants them to be. And some of the potential warlock patrons named in the official D&D rules actually appear in person in official D&D adventures.
Also if you want to be a super stickler to the rules warlocks do exist as an official D&D 5e monster you can encounter, as published in Volo's Guide to Monsters, an official WotC publication.
https://dmdave.com/warlock/But as always, DMs are free to power up monsters to be viable as enemies for higher level parties, or create entirely new monsters and NPCs as they see fit. There are no rules or limits dictating that DMs are not allowed to create powerful warlock NPCs...they can literally create a warlock character the same as you create yours and can use said warlock as an NPC enemy for the players to fight.
