Complete shutting down of Internet can destroy bitcoin, of course many user will be unable to login their wallets, keys will no longer worked, their will be massive breakdown and lockdown of exchanges, no transaction will linked up e.t.c.
There is no reason why internet will shut down in years ahead may be in event of war when there is massive destructions of Internet equipment and facilities.
I think maybe just a war that can make the internet around the world destroyed. because the main connection bitcoin is the internet network, if there is no internet then we can not do bitcoin delivery transactions. so nothing can affect bitcoin other than internet. all networks in this era of technology must depend on the internet network.