And why is a hydrogen economy a good idea? To reduce CO2? Is CO2 a threat?
How do you know that?
Is CO2 a threat?
No, it's not. NOx, SO2, Ozone and all the other stuff that comes from carbon combustion can be a problem but with the exception of environmental hell holes like India or China or Mexico City, I wouldn't even consider them a "threat".
But once again, what does that have to do with the potential economics of using H2 as a transport fuel? Why have you made up your mind that it is impossible?
A little over 100 years ago, people likely thought the idea of a global petrol-based fuel transport system was nuts. Where would we get all the oil? How could we distribute it? What would we do with all the "useless" hydrocarbon by-products of refining? Trying to think through all those technical and logistical problems in advance had to make their heads swim. No one person was smart enough to figure all of that out so some who considered themselves smarter than others probably said "it will never work and we shouldn't spend any money on it."