There have been other widely supported changes to Bitcoin that have been implemented, so surely this can't be the strongest argument or the best steel man of those who oppose it?
I think it is. If you can think of a stronger argument I'd like to hear it.
How does testnet reveal potential issues related to the economics and game theory of the change?
afaict BIP-300 does not affect bitcoin economics or game theory for any bitcoin users who aren't using a hashrate escrow, so to everyone who doesn't plan to use a hashrate escrow, the most important consideration is whether or not the implementation is technically sound.