[p_l]

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.

[arghwhat]

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.