IMO Bitcoin will die when fiat dies

Right now, much of the usability of Bitcoin is built on people being able to easily transfer it into fiat to pay their suppliers

Even if your supplier is Bitcoin friendly, is their supplier?
You need a whole circular economy to keep using Bitcoin in the absence of fiat conversion 
(or assuming that in the event of fiat collapse much of the world returns to metal disks, you need a ready conversion mechanism between Bitcoin and metal disks)