Personally, I think this sort of decision needs to stay in the hands of witnesses, where it is public and transparent.

The witnesses answer directly to the community, via stake-weighted voting.

This is exactly as it should be, imho.

If @gtg lowers the APY and enough stakeholders unvote him as witness, he will reconsider.

Also, keeping important parameters at the witness level allows extremely quick responses to attacks and other potential problems.