Chiming in from the perspective of someone integrating into the protocol. I do think the liquidity mining helps grow the total pie and can bring meaningful amounts of new capital.
There is currently a few community members working on an integration between PoolTogether and Aave so Aave powered prize pools can launch. PoolTogether currently supplies ~$130 million of supply side capital to Compound Finance. Having a high effective APR means Aave should see similar if not greater amounts.
Additionally, it doesn’t direct result in selling pressure. PoolTogether basis on yield farming rewards directly to the prize winners so it’s up to them whether to sell or hold those.
tl;dr I think this can be positive sum and grow the total pie for Aave.