I didn't know there was an issue re replacing all the existing equipment but I'd always understood [from when this question had been asked previously
elsewhere] that AM had been retained, as opposed to anything more seemingly sophisticated eg SSB or FM, to maximise operational safety and reliabilty-in the same way that the english language was standardised.
It's not hard to imagine the difficulty involved in trying to achieve Air Traffic Control within a variety of reception conditions, especially on the ground
I can't claim any specialist knowledge though.
Dave W