According to this NPR article there are 4 million flights under 500 miles in the U.S. alone. Based on my quick estimate of the chart in the article, that’s about half of the flights made. Other parts of the world may be better or worse depending on terrain, etc. The number of short flights is going down due to efficiency. They mention pilots, but everything else indicates fuel costs. If these hybrid planes took over those flights, and are as effective as projected, they could maintain equivalent or better fuel efficiency on short hop flights, making them more viable.

Now, the point about trains is quite relevant, but the infrastructure for planes is already there and I don’t see regional train routes filling that gap any time soon. This would also make synthetic avgas more viable since you would already be using less overall.