This is how you make magic special in your games: Emphasise the cool plot-worthy items give them grand names and deep histories that the PCs can research, and don't make a fuss over the bags of holding and rings of swimming etc that can often make up a treasure hoard.1...
So other than the fact that the bag of holding is a smaller space, there’s ample precedent that people can live in extra-dimensional space for short periods of time. If the GM wants to say there’s only a certain amount of air, so you can’t stay in there for long, that’s ...