In my opinion, they are thinking like that because they dont have the knowledge about our modern technologies today, most of the people who are criticizing cryptocurrency are those people who are old enough that they cannot understand how to operate a simple ipad, so they literally dont have a clue about the way our system is going.
Yes, the older generation are unable to understand crypto and therefore they are not sure if it can be sustainable in the future. I think we should give proper understanding and knowledge to these people so that their interest in crypto can be developed and they can also use and earn good with crypto currencies.