D&D 5E - Magic Jar aftermath question

The problem of DM fiat is it breaks the verisimilitude of the fantasy world, is bad writing when authors do it, and foils the fun of the players' creative attempts to solve problems within the rules.
Different perspectives I guess.
Verisimilitude is broken for me with the assumption that everyone everywhere has access to all the same abilities and spells exactly like everyone else. That doesn't make sense to me at all that there would never be anyone ever out there who had different abilities or new spells.
Why is it bad writing for a bad guy steeped in evil to have access to magic that no one else has or that he's taken farther than anyone before? Were Horcruxes in Harry Potter then a bad plot device just because the heroes didn't know how to make them?
I'm all for creative roleplaying. If the bad guy has something that no one has seen before, maybe that is a good plot hook for research or questing to find an oracle or... I don't know.
I'm not saying "let the guy do whatever he wants and there is no way to undo it ever". If you're going to present something non-RAW then you as the DM need to also know how the PC's can solve it... if you want it to be solved and not a narrative beat you're trying to hit.
Sometimes not everything has a solution, sometimes you lose. And that can be powerful RP opportunities as well, to know that your friend is truly gone and there is nothing you can do about it....
Perhaps our villain used the magic jar as a temporary solution because he wanted to use the NPC to spy on the party and also needed time to prepare for his final transfer to the clone?
Sure, I'm all for that! RAW, It would have just had to have been a cast/created Clone and he'll get to it when he wants. It sounds like a great way to do it. It doesn't solve the OP's desire to make Magic Jar permanent, but it looks fun/cool and would make a good story!
Hmm, the villain has an ability to make clones. Can you ever truly kill him?
Yeah... that was the 2e/3.x Manshoon issue in FR, he had too many of them around.
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