By the time any major high speed rails are built, electric planes will exist.
Given electric planes can be fueled with green energy in many European countries there is not really an environment argument anymore. Well one could argue that its less efficient and that true but the difference is certainty much closer.
Electric planes will also make air-travel much cheaper, because the operational cost will be far, far lower just as with electric cars and the 'fuel' will be cheaper as well. The fuel is consider a major part of the cost right now and has the potential to be much lower.
Electric planes also require little buildup of infrastructure. In fact, electric planes will allow us to fly many more routes to smaller airports that are often not utilized much right now.
If we can get the absurd security restrictions removed and make air-travel within continents more like trains we can eliminate much of the wasted time as well.
Because electric planes have the potential to be much more quite while on the ground and on landing, airports can be build closer to population centers. And there are additional innovations that could make propellers in general more quite as well.
Planes allow for point to point transport rather then following a line of stations. Or rather you still might use stations but on much less granular scale.
From where I live I can reach a local airport in something like 10min with the bus. If I could fly from there to places like Paris, Milan or Berlin it would be quite amazing. Even if those flights only happen once 1 week or so. Or you just connect to some larger airport and get on a second plane there.
I feel like really major investment in high speed rail is misplaced. I much rather see more investment in local rail. And if you need to hope from city to city jumping into an electric plane would be better.
Now the technology is not fully there yet, specially in battery cells (and how to package them into plane). No proposed electric plane can fly Zürich->Berlin. But I think major investment in that sector would yield amazing results. The current electric plane companies are minimally innovative because they want to just get electric plane of the ground (and they are right to do so).
However once you really think threw all the implications of electric planes you could do so much more. Its like first generation EV were just traditional cars with batteries in them. When now in Tesla and BYD produce cars where the actual battery cells are a vital part of the cars structure.
With battery electric supersonic overland flights are a real possibility because you would fly high enough that air density and distance would make the sonic boom much less of a problem. We could rethink vertical launch as well.
In general a change in paradigm will force revising on many of the most basic assumptions of how traditional combustion planes look.
I would not be surprised if we fly around with electric vertical starting, supersonic flying wing airplanes in 30 years.