It's also perfectly valid to criticise the criticism for having too high standards. If you expect a car to fly you to the moon, your standards are too high. If you expect Reddit to be like HN, you're also a fair bit off.
No, it's not really valid to come back saying that nobody has done better so therefore your sights are too high. If someone is complaining about the CO2 output of the lowest emissions coal plant, it's a perfectly valid criticism even if it is the best coal plant.
Reddit is a cesspool, and there is no proof that's the only thing that can happen with large online communities. There is plenty of room for innovation there. It's not like online communities have been around for thousands of years and this is clearly the best solution as proved by extensive research.