Once cryptocurrency is universally used, it won't make sense to keep it as an investment as the price will stabilize.
Quick question op, are the Fiat currencies that are universally used stable?. Bitcoin won't give as much ROI as it usually does holding so the only option will be trading just as the fiat currencies are traded between themselves in the forex market, so will bitcoin and other cryptocurrency be traded. That's why bitcoin is the solution, it's decentralized, digital, can be used as an assets also used as a currency at the same time.
Stable enough to be trusted to hold a relatively steady value and make transactions with.
You're right about bitcoin being the solution to be used as both a currency and an asset. This won't change, but it won't have as much potential as an asset if it's value stops moving so much one day. Its ultimate use lies in buying things with it, which we have yet to witness on a large scale.
What I'm trying to say is that this will lead to a digitized future where maybe consumerism and buying things (100-200 years) might not be apart of everyday life.