The current revisions of ED-23 and DO186 still count the aviation NAV/COM bands as 108 to 154MHz.... but don't tell the inhabitants of above 136.975MHz
The 'FM advantage' with 25kHz channels is not going to be a lot, but it was enough to convince maritime and PMR users.
Fortunately the aviation lot when they were feeling the pinch on space didn't opt for grabbing more MHz, but opted to split some (not all) of their existing 25kHz spaced channels in three. NBFM packed into such a narrow channel has no real benefit over AM, but the not to change to FM decision has been made and remade from the era of 100kHz channels and onwards.
Tests have been run at various times to compare the two formats. I think the possibility of changing was last seriously considered in the run up to the channel split from 50kHz to the 25kHz '760 channel' plan. The people I know on both committees weren't involved back then.
Anyway, I wonder how many other people in the 21st century have designed and put into production AM transmitters?
David