There's cost in simultaneous replacement for huge portion of the fleet.
Don't devolve into simplism, consider that you need to replace the radio for everyone sharing the same space, and that there might be way more planes sharing that space than you think.
The cost of replacing equipment in a fleet is large, but the modulation no longer has any impact on that cost if a replacement was to be made.
AM is not providing any benefit of simplicity, but not changing standards avoids the transaction cost of change.
And who would pay for the Supplementary Type Certificate for every single aircraft model out there, including many that were built by manufacturers that no longer exist? I don't think you understand how this stuff actually works.
Equipment certification for aircraft no longer in production is not at all related to the simplicity of AM. Certification complexity would have been exactly the same if FM had been selected as the standard back in the day.
That is not a factor anymore. Capable wideband transcievers like the ones in Baofengs and similar supporting multiple types of modulation cost cents.