You do know we've basically been given unlimited wealth, plus an army, or what have you. So we can already churn out tons of magic items, if that's what we desire. Though obviously not in the same manner as what Sparel is suggesting.
Besides, I believe only he truly benefits, as he's the only immortal one, in his narrative of course. Not like he has a slew of immortal craftsman, totally dedicated to magic item creation.
For him to really take advantage, he'd need to spend most of his time in the demi plane, creating magic items. Which I not *think* he plans on doing...
He can do this outside the flow of time. He can spend nigh endless time doing that there and no time will pass in the real world for him doing so. He could note that he spends decades on end at that and only need to just note that, as no actual time will pass because he does it all during time stop.
It's, even with followers and armies and what have you, more than a bit much.
Neverminding that there's no plausible reason everyone else who could do it, which are a lot of people even just in the world, wouldn't do it either. It opens a door as far as just simple setting logic that doesn't seem like it should be opened. There's stuff you can go "well we can just say that doesn't happen" and then there's.. this.
It just seems like it's too much.