Meantone is a modification of three-limit just intonation to try to better approximate the thirds from five-limit just intonation. I know that it is historically inaccurate to say that meantone follows from five-limit, but in order to understand the point of meantone's existence, you have to know about five-limit as the standard that meantone is aiming for. I should probably rephrase my post to say that meantone follows logically from five-limit, not that it follows historically.

All humans (and some monkeys) can detect octaves, but that does not mean that all humans consider octaves to be equivalent. That's a culturally specific convention. It's a very widespread convention thanks to Europe's wide cultural reach, but it's very much not a universal one.

Split keys may have been common for a while, but I think we can agree that in the long term, they did not catch on.