Este Nuno, kbm
It's not completely unreasonable. The alt-coins dilute the value of the accepted currency. In other words, when they are first created, they have a comparative trade value which over time is either endorsed or abandoned, just like a foreign paper currency on the monetary exchange. When a foreign currency implodes, it drags down the value of all the other currencies because that currency was traded away for realer goods, which raises their relative value against all currencies. We'll use BTC as the example for illustration:
A new crypto-coin is announced, it is mined and exchanged (ie: dumped) for BTC establishing a relationship of relative value. Now, what happens when that alt-coin dies? This is the downward pressure we see in the price of BTC. The closer we move toward a global economy, the closer we move toward a natural monetary monopoly and if the world population decides that crypto-coin will be the preferred medium of exchange, why is it unreasonable to assume that one surviving coin will be it?
So the statement then is more that bitcoin would be reserve currency. While I wouldn't mind that at all, due to owning bitcoin, I'd have to make the point that there are still massive political and physical barriers that need to be dealt with that people are still grappling with today (I believe BRICS just met and formed something to bypass the fed pretty recently). The only way the current reserve currency for the world got to its current place is with guns and resources. Crypto doesn't have guns. Who is going to support it as a world currency? There's no political affiliation therefore nobody is standing behind it with guns and resources saying it's going to be worth a lot throughout your lifetime.
What does crypto offer? Lots of hopes and resources saying it's going to be worth a lot in my lifetime. No guns, or imposition of will. Only investors, and their word that it will be worth more next year. I'm not currently convinced that without some serious political endorsements (not just regulation - and more like the entire country of Argentina) that any of this will ever go anywhere unless some country with a metric shitton of both guns and resources happens to start using it. (edit: anywhere besides lowering transaction fees to almost zero -- that's still extremely valuable and was the main reason I've invested.)
But which one of them would let themselves be at the mercy of the various mining hardware scattered across the entire world (mostly not in their control) that's required to get that sweet inflation in their money supply so they eventually don't have to pay $.01 for 50 loaves of bread? Who would be the first to jump on that train? This is the basis of my statement - nobody would voluntarily do that, they would make their own fork and run on that chain, where there would be a very large amount of niche-filling currencies.