In an abstract way: we are all part of a gigantic hydrocarbon economy. We are carbon based life forms, we're everything absorbs and emits CO2. We need hydrocarbons for almost everything we make. Why would we create a new hydrogen economy? Does it have any use except for energy storage?
Elon Musk is sceptical because he thinks about the fundamental physics behind technology.
Hydrogen is a gimmick which plays on a knee-jerk reaction against CO2, and a very short sighted view which assumes the that batteries won't get better or that we won't get efficient fuel cells for hydrocarbons and CO2 neutral production (while assuming a breakthrough in hydrogen fuel cells).
If you want to predict the future, you have to consider physics, not what's cool (the exhaust is water! )
In this case fuel cells seem to have the same issues they had 20+ years ago when they were touted as the feature.