You can say that about any currency. An 80 year old billionaire still needs someone to care for him, just like an 80 year old pauper. He relies on the hope that his dollars will be accepted, to look after his medical and care needs as well as security and ownership needs.
My understanding is that because people have mortgages in dollar (or other useful money), you can be pretty confident that many people are going to be motivated enough to get some dollar (they have to pay their mortgage with it)
On the other hand, no one is going to be in a difficult situation if they cannot get bitcoin.
Well the whole idea of Bitcoin being a store of value isn't that its price is stable between 2017 and 2018, but its non-inflationary in long run (something which isn't true for the time being, it is inflationary.
However, the idea is that between 1971 (the closing of the gold window), and now, USD and all fiat currencies lost 99% of their value. The same is not true for Bitcoin on a similar time frame (i.e. 30+ years).
The US government is functionally bound to the dollar, and its size is enough to guarantee the continued functioning of the dollar. On the other hand bitcoin is backed by speculators and criminals, hardly a stable foundation for a currency or commodity.
