Steven P. McNicoll writes:

Wouldn't that reduce the available frequencies?

For a given audio bandwidth, FM tends to require somewhat more radio
bandwidth, as I recall, but the audio bandwidth of aviation radio is
already so limited that I don't think this would be an issue. The
gain in clarity would outweigh any loss of audio fidelity, assuming
that the same channel widths were used.

If frequencies were reallocated (instead of allocating new ones), that
would be different. That would also obsolete older equipment much
more quickly, which might not be acceptable. But there must be some
space somewhere that could be added to the frequencies, or perhaps
some band so rarely used that it could be reassigned.

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