It's situational. I know that. But that's why it's fun to come up with these really weird build ideas. This one just so happens to be more effective against the players than the enemies they fight, for the most part, part of why I noted it would make a great villain.
Again, true. That's part of why I selected Hypnotic Gaze as the ability that's an ideal combo, since you can also incapacitate them indefinitely, provided you forego an action each turn. Can't do any support or AOE if they can't act.
This is true, although ways to do this in combat are limited. The torpor poison is the only non-magical way to do this if you don't count unconscious as being asleep (which, technically, is a better description), although all 3 other poisons inflict this condition, as well as Eyebite.
It's true you can just wait for the target to be asleep, but the point is that you can do this mid-combat over 2 turns and there really isn't much anyone can do about it.
The charmed condition from Create Thrall lasts until otherwise removed, the ability is used on someone else, or a Remove Curse is cast upon the target. These abilities have no limit on daily use other than Hypnotic Gaze being once per creature per long rest.
Importantly, Create Thrall's charmed condition does not break under normal conditions, like taking damage. And if you've reached a level 14 warlock, you likely have a 20 in Charisma, which, if you have advantage on attempts to persuade the target to not attack you at that point and proficiency, you're likely going to succeed.
Finally, putting a few levels into wizard gives a warlock extra spell slots for longer encounters, which can save them. They don't want to be using 5th level spell slots on something like, Silvery Barbs, for example, despite how good it is. I'd argue taking 3 levels in wizard is even better, since your potential spell slot access doubles.
This is very much a build that can take down major threats single-handedly, and is designed to function as such. A little bit of a meme build, but an enjoyable thought experiment.
I came across Create Thrall when I was making my character for Curse of Strahd. I decided to go with Goolock for thematic reasons, and made a list of every way to incapacitate someone in 5e, even knowing the campaign doesn't get to level 14. Hypnotic Gaze still stands out to me as the clear winner for being recyclable and also a really strong ability on its own, as well as not (RAW) breaking Sanctuary.
I still think Goolock is weak (this can't affect non-humanoids, which I find a little disappointing), but nowhere near as weak as before. Honestly, this could make a GREAT villain.