I find this logic sound, but incomplete.

All of the prices of current coins are already a speculation of their potential.

e.g.Ethereum, it might have real use and potential. But the actual price already reflects that. Nobody's paying 270 usd per ethereum thinking it might not have a real use (ofcourse outside speculators). People are already paying that sum because they believe in a huge potential. But that doesn't mean that in few years Eth's real value will reflect 270 usd per ethereum. In fact we may find that even with a real use, it was much overpriced june 9 2018.

Also, another example: Ripple.

It has a real use, true.
But who prevents banks and institutions to copycat Ripple's idea and code? (which is partially open source).
It has also this huge problem with the XRP that the founders have.

So, you see again, Ripple has real use, but also a lot of problems.