15 years is a very long period of time. During this time, everything will change rapidly. Perhaps the best investment will be gold or real estate. As for crypto, it is necessary to pay attention to coins that have their own blockchain. Great prospects have such coins as Stratis, Eos, Aion, Icon and the like. And of course - the Ethereum.
I agree with you...i allways say if somebody want coin for long term need to focus on coins-tokens or ICOs that are developing own blockchain, especialy for those which are developing som inovacy which other blockchains do not offer...For example: ethereums inovacy was that started to offer smart contracts based on blockchain - ethereum was first which beginned with it - this is the reason why is now so high....
who knows what kind of innovation will be successfull....there are many projects developing blockchains 3.0- every project try to bring somethiing new: for example quantum proof, decentralized app ...it is really difficult to say which kind of innovation in blockchain will be able to attract big attention of investors and will be considered as blockchain 3.0 .....