If I've understood your question well you ask for stability, reliability and a good investment long term?

As cryptocoins are based in code in my opinion they won't be a safe investment long-term, 20-40 years for something like a retirement fund. All things related with computers and programming are highly mutable and fluid. Just look how has changed the pc world in this last decade. As new hardware appears new revolutions can change it all, old things that seemed reliable could become obsolete in a couple of years but same as the are risks there are many great opportunities.

So if you want to buy something and forget about it for the next 40 years I don't recommend cryptos. But if you're willing to keep an eye to your investment, search for opportunities and take risks cryptos are a great place to look. If you want safety and a bit of profitability long-term look for gold when it goes back to 400$-500$/oz. or invest in diamonds, something tangible. And always diversify your investment.