Personally, I don't expect die-hards like Warren Buffett to get interested in cryptocurrencies any time soon (if ever)
Assuming he genuinely mean what he say in interviews, Buffet's "Bitcoin is an asset that produces nothing" is probably how most of the older generation of investors see cryptocurrencies. They'll obviously be more conservative with investments - Buffet regretted not investing in Amazon for example
Buffett invests mostly in real sector
Financial instruments where Bitcoin truly belongs to are of no interest to him unlike, for example, George Soros (who is a dyed in the wool speculator). And in this manner, he is technically right about Bitcoin producing nothing. He means that Bitcoin doesn't produce goods which can be consumed and that point can hardly be challenged
On the other hand, Bitcoin is a payment system (at least it is designed to be one) and as such it can be considered a service helping people move money to and fro. And in this sense, its value is as real as anything else out there (provided people actually use it, of course)