Interesting Perspective, I like your points. Also, It would not "only be around for 30 minutes". It would remain at your side until its defeated, or 2hours have passed, as with all Current Jug pets! It would basically be a Slightly more powerful jugpet in which you could customize a bit.
However, The idea of an "Uncharmable" mob as a Jug pet stems from the idea that uncharmable mobs will likely never become jugs, as such players will be far less likely to control them. There are some mobs that are Charmable that should become jugs, I would rather see those added as a Recipe though, not a quest! Where as the ones that are generally Strong-willed Creatues (Uncharmable) could be raised from younger stages to become friendlier, allowing the summoning of them. SO basically what I'm trying to say, I Agree there are some Charmable mobs that would be wonderful additions to our Arsenal, But Thats what Level 91-99 Jugs would be for!
This pet would be more of a "Special Summon" Type deal. a Slighter Stronger version of our current pets, Summonable similar to how you would summon Alexander/Odin on Summoner. Which is why i Suggested also instead of a New Ability, Augment Familiar to summon it, Thus keeping it "Unique" for its power.
The "Raising" Process can be reduced to ~10 days, ~5 Days, I wanted to maintain some interaction with your pet, to actually create the "Bonding" part, also to have some control over its development and stats in a small way.
Now, This pet would not be indestructible, It would be more powerful than our current bet, But not so much to obsolete them.