Leaders like Trump think they can control the flow and volatility of Crypto currencies just be sending out one tweet. In a way they are scared about how this new form of digital money is giving power back into the hands of the general public.
The mere existence of crypto currencies is a threat to governments and banks because they have a feeling they will lose control over the population and how and where they spend their money. Banks regulate our funds according to their needs and with the emergence of crypto currencies, controlling the spending power of the public has become an issue for banks
Well, there are different options (as well as opinions)
And Trump is known to keep his open leaving him enough room and space to maneuver. You couldn't actually expect him to be any different given his business background. He is totally pragmatic, through and through, even if that requires him not to look like he really is
What I mean to say is that he doesn't burn bridges behind him, so we shouldn't really expect him or someone like him (which is the majority of the power elites anyway) to outright ban crypto. More likely, they will try to weaponize it and use it for their own purposes and agendas (whatever those might be)