You cant do that by buying cryptocurrency, it is more comparable to buying some new exotic raw metarial, and hoping someone will find it useful at some point, which I find highly unlikely. (Not blockchain technology, but cryptocurrency)
Yes to some extent you are right. Bitcoin has been compared to gold. if you think of crypto as gold but there are no gold smith you are investing into future adoption. and once the adoption is here and demand increases so will the price of your gold.
And there are actual potential use-case most obviously in finance. stocks could be NFTs and could be exchanged by anone on decentralized exchanges for close to no fees (On Layer 2, fees will be minimal, they are already now, search for loopring).
One could also imagine a decentralized "kickstarter" and you can be part of it with just $100. Which us impossible right now with venture capital.
Then the whole decentralized part, not being able to be shutdown or blocked by the goverments. In that case crypto would simply be a different currency and the ideal one obviously for this job is monero. For me personally this is the most important part. freedom. But of course its hard to give that a specific value.
See also
Note: I don't actually own any ada.