I aree with everything @sjors has said.
I don’t see any reason 600 should be used.
The onus shouldn’t be to prove there’s a danger. We know there’s a danger to more restrictions breaking something.
I don’t understand why 600 was proposed. How is being more restrictive beneficial?
Bitcoin should be like a rock upon which developers can build without getting rug-pulled like a Facebook API. Microsoft build its profitable evils on the back of a similar rock: every developer’s program since 1981 still runs on it.
If you break someone’s software, you destroy far more systemic bitcoin value due to perception than the harm to the victims.
I regret giving walls of text on theoretical ideals if it distracted from @murch 's wise recommended fix that is the least restrictive and therefor safest. It’s not even the same type of change as this one which has to specify a somewhat arbitrary number.
In the distant past nullc’s (even wiser?) recommendation on timewarp was “don’t touch it”. His argument was that it can be easily fixed if it happens and there’s already a non-fixable fundamental break in the security (>50%) if it does. On testnet it’s more of a risk that needs addressing. With a some imagination, leaving it alone on main chain could be a honey pot you want to keep.