Comment by tenpies
I was thinking about this too.
My most likely "internet down" scenario would be my local government deciding to launch a national firewall, probably under the guise of "combatting disinformation, malinformation, misinformation, and foreign interference".
For that scenario I need as many VPNs as possible, a VPS in a friendly jurisdiction, and a TOR browser.
If the "internet is down" for "good", it's hard to me to think of a scenario where it didn't bring down the rest of civilization down along with it.
In the U.K. a Thames barrier failure would (amongst a lot of London) wipe out the vast majority of switching capacity. Sure there’s slough and a few smaller IXPs but not enough for the capacity.