In MWO, the economy is thrown out, so maybe create another economical reason to use them: Cbills earned.
Make medium mechs the best Cbill-earners.
Either nerf current heavy and assault payouts, so that only medium mechs get the current cbill payouts, or...
Double current cbill earnings for medium mechs (and leave others the same).
I bet you'd see a lot more medium mechs on the battlefield.
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