this can turn out for them, at least, a waste of time, and at most, a loss of funds due to false perception and skills acquired on a demo account.
Different people will have different opinion but something must be better than nothing; so having demo accounts will help to get some basics of trading before touching live markets rather than directly entering to crypto trading. This can be demonstrated like this: demo accounts may not guarantee you any profits but definitely will help you to manage your emotions and risks for the same markets (means practicing in forex markets and then trading in crypto markets will nullify the point of demo accounts).