Why can't that stuff just remain diesel? We can synthesize it, no problem.

As I said, 1%. And in those cases using hydrogen makes little sense.

Emergency vehicles of most types don't need that however. And in many of the cases you mention swappable battery packs are an option as well.

But we have to realize that compared to all consumer and fleet cars, 99% of all trucking is the vast majority of fuel use in transportation.

Extreme long range trucking is the exception.

In most cases you would make the hydrogen into methanol and you can keep mostly the same engines.

Most hydrogen is produced from methane and is actually worse than gasoline for global warming because methane leaks and is a far more potent greenhouse gas than CO2.

It makes indeed more sense to just use renewable gasoline, e.g. using the Fischer-Tropsch process like VW's eFuel.

I know, but its easier to go from renewable hydrogen to methanol then eFuel.