It makes sense that domestic flights are dropping. They already mostly disappeared decades ago for individual/pleasure travel. Taking the train, bus or driving is just better value. So it's just business travel left. I used to occasionally travel between Gothenburg and Stockholm (the two largest cities) for work for a while. Flying was slightly quicker, but only barely. There are non-stop high speed trains that take you directly from center to center. More pleasant too, and you can get some work done on the way.

An airline ran TV adds pitching that flying gets you home to the kids earlier. Basically pitting peoples guilt about spending time with their kids vs. guilt about the environment. I guess it didn't work. The same children will inherit the planet after all.

I'd be surprised if international flights have dropped much though. Taking the trains any further than Copenhagen doesn't really work, unless train travel is the main point of the holiday.