There are different reasons for the decline of skiing in Germany. One is the climate. In the last 10 years the most winter were very mild with not much snow. A huge difference to years when Sommerfeldt, Teichmann or Angerer growing up. In general you can do cross country skiing only in mid mountains. Also there is not to much interest for this sport. Dont know how many kids didnt even knew these athletes.
Most of these sportsmen in cross country skiing and biathlon where spotted and promoted in East Germany too. Obviously sport had a much higher importance for GDR. The support for young athletes was one of the best in the world.
We are seeing the same in Norway with much shorter winters, but cross country is probably as popular as ever. Both in terms of people going themselves and the attendance at events. Before Covid the Holmenkollen-weekend was insane. Racers were complaining about losing their hearing during the race - and not at the stadium, but at spots out on the course.
In terms of getting people interested cycling, cross-country and I guess golf have been by far the best over the last 20 years. Golf is a bit different as it isn't a big sport among kids, but cycling and cross-country is (CC much much bigger than cycling, but often something kids combine - Fossesholm as an example is also among the world's best MTB-cyclists).
Even here for most of the year cross-country skiers train mainly under "summer" conditions. Not sure if that in itself is that much of a limiting factor. Of course it helps being on snow for at least some months, but the main part of the training for kids would be without regular skis.
I think you have to make it "sexy" somehow. I know a lot of people both in Belgium and Holland having spent many years working there - and so many of them will go to the Alps to do alpine skiing. They are probably all better than me at it - even if I grew up just kilometres away from a hill. For Norwegians you might take your family to the mountains to do cross-country. That is what happened when I grew up. We went more often to the mountains to do cross-country than alpine skiing.
In Norway cycling have managed to develop a new "sexy" culture. When I grew up hardly anyone had road bikes and the idea of racing on bikes was completely foreign to us (at least road - one guy I grew up with did MTB racing). Now you see road bikes everywhere - even if the season here of course is shorter than in most other places.
Time will tell, but it seems like cross-country is a dying sport. I do think it is possible to "fix", but not with the current leadership in FIS etc.