I didn't say anything about Magic Jar.
Wish lets you duplicate the effects of any spell of 8th level or lower, ignoring expensive material components and long casting times. So long as you do that, you avoid any of the potential drawbacks (necrotic damage, 33% chance of never being able to cast Wish again, etc.).
Clone is an 8th-level spell. You don't need the cubic inch of flesh. You don't need an hour of your precious wizard time.
Demiplane is another spell you can do with Wish, or cast it on its own right. So you can Wish up a demiplane, sit in it for a day with some rations, and start Wishing up some clones of you and your party.
Once a cloned creature dies, its spirit goes to the cloned body with all its previous abilities and statistics. The clone can be a younger version of the creature too, so goodbye dying of old age.
As for getting out of the demiplane, as long as you have Wish you can Plane Shift (without the required tuning fork) to get out and back to the material plane. And because you are mentally and physically identical (save for age) in your new clone body, you have the same spells prepared.
This is a pretty well-defined, RAW-permissable exploit, and it's one reason playing max-level games in 5e is ridiculous. Potentially fun, but definitely ridiculous.
Of course this can all be avoided by banning Wish as a spell, since that gets rid of basically all the easy exploits by Wizards, arcana clerics, bards, and sorcerers, while leaving their 9th-level spell with some good options anyway.