Gas costs should inhibit parasitic farms from attracting liquidity. They would need to attract sufficiently large TVL from sufficiently large set of users such that:
(1) the gas cost of rebalancing/rotating their funds < yEarn gas cost + YFI fees, and
(2) their farm has enough interactions from users to constantly trigger the rebalncing/rotation and achieve the same optimality as yEarn.
Recall: yEarn contracts are “asleep”, they don’t just rebalance/rotate on their own … they only do so when a user interact with them in some way by depositing/withdrawing etc.