Rémy ,

IIRC it has already happened. There was talk of changing from 25 to 12.5 to double the number of channels but instead they went straight to 8.33kHz effectively tripling the number. The change to a long time to implement with both ground stations and aircraft. The scanners (particularly handhelds) available around the time could switch to 12.5 spacing and still pick up plenty of signals, courtesy of breakthrough due to the dreadful filtering they used to have. We did some tests when I worked for Lowe Electronics, we were right under a major air lane, Amber 1 I think.

Perhaps filtering has improved  in the day of the Ultralight. I don’t have one but even if I did, the skies are quite empty above me are noticeably empty.

HTH

73 Tom G6PZZ