So, you believe in the Labor Theory of Value (
Labor theory of value - Wikipedia). That the value of a commodity is dependent on the effort it took to produce it, as opposed to its actual intrinsic/inherent utility. A cell phone built by hand, a process that would consume hundreds of hours in an un-automated factory, if it was possible at all, is worth more than an automated-factory-produced cell phone, simply because it was harder to make.
Have these complicated mathematical riddles made a Mars mission easier to complete? Why would a "miner" be rewarded for solving a complicated mathematical riddle that does not add value to anything outside the bitcoin once it is solved? I have never seen an argument for the practical utility of solving complicated mathematical riddles for their own sake, any more than it makes sense to take a shovel and dig a hole in the ground, and then fill it back up -- it was hard and took a lot of work, but what was actually accomplished?
So again, what makes Bitcoin valuable, aside from the demand for Bitcoin produced by Bitcoin? This is not an argument
for a fiat currency -- it is an argument that Bitcoin
is a fiat currency, albeit one without a history of legitimacy.