Even Zimbabwe banknotes are worth a little money.
Not as currency, of course; Zimbabwe itself switched to USD years ago.
But people buy them on eBay regardless. Why? Well, because it's an interesting object, a funny gag gift, or a conversation starter.
So let's say, for some reason, that one day people stopped using bitcoins. Stopped accepting bitcoins as payment. Let's say the big mining pools all close up shop because it's impossible for an unused currency to pay their electric bills.
Then, even in that world, I imagine the system would probably still live on in the same capacity that Zimbabwe Dollars have survived. Folks buying mining ASICs on Craigslist for a pittance and running them out of nostalgia, or because it's funny, or just because it's something to do.
So we return to pre-2011 prices - sub-$1 prices - from back when BTC was just a funny crypto-nerd novelty. It won't be worth anything as an investment vehicle.
But that's not zero.