I think the argument that Bitcoins will ultimately have some nonzero value is fairly sound under certain assumptions, namely (a) the crypto doesn't get broken;

I think the argument that Bitcoins will ultimately have some nonzero value is fairly sound under certain assumptions, namely (a) the crypto doesn't get broken; (b) governments don't outlaw it. If Bitcoin turns out to be useful, people will want to keep a certain amount of it on hand so they don't have to continually trade in and out of it, with the attendant fees. That doesn't mean, however, that a bubble hasn't formed or will not form. Speculative bubbles take on lives of their own, and can drive the market price well above the true value, as we've all seen.