Australia

The first two and last bullet points might apply to motor vehicles, but the third and fourth certainly do not. Anyone claiming that it's not safe to operate a car at 30 km/h is just lying. There's just no way to make that argument without deliberately saying something that you yourself know to be completely untrue.

for what is apparently a busy shared path 20km/h seems a more reasonable limit) rather than either removing the limits or saying you can’t fine riders for exceeding them

The thing is, the Goodwill Bridge, at the other end of Southbank, is 20 km/h. But it's very notable that this is still inappropriate there. While 20 km/h is a reasonable speed for someone on an upright dutch-style bike riding on the flat, the Goodwill Bridge has rather steep inclines—and, more pertinently, declines. Staying under 20 km/h on a road bike on a fairly steep downhill is not much less ridiculous than the 10 km/h limit of the Kurilpa Bridge.

But then it comes back to the simple matter of logistics. Cyclists aren't required to have speedometers, and while some sports cyclists have them anyway, they usually rely on GPS which is much less accurate, especially around the inner city, than the direct-drive speedometers of a car. It doesn't make any sense to enforce a rule where it's impossible for the culprit to know they are in violation.

The fact that this conversation is even taking place is a sign of some pretty severe car-brain on the part of people who support the speed limits. Speed limits aren't in place for cars arbitrarily. They were created because cars are really, really dangerous. Cars kill hundreds of people per year in this country alone. Speed limits make sense, because they're one of many tools to help keep that number from getting even higher. No evidence exists to support the need for speed limits on bikes. The only argument that is ever made essentially boils down to "cars have them, so bikes should too."