AC already does this for economy fares by restricting mileage earnings on Tango fares vs. Flex and above.

UA offers its own FFs 100% redeemable mileage earning based on distance no matter what the economy fare class. A nonsensical practice (from a business perspective anyway).

AC could of course follow what UA/DL have done, but already does in a rougher sense by restricting Tango earning. Which is more akin to the European and Asian carrier method.