First of all, the "zomg portals are closing" thing has been done in at least one of the official Planescape campagins.(It might have been Faction War, but I'm not sure). Secondly, creating a whole new plane "Netherspace" is pretty much disrespecting the source material, especially when it's completely unnecessary.
Sigil is called "The Cage", because you can only enter and exit via portals, no other means of transportation work.
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Apparently you're the romantic and I'm the realist.