Its a logarithmic scale, it makes sense that 10 is the maximum. The difference between warp 9.1 and 9.2 is the same as the difference between warp 1 and 6, for example.

It’s like decibels, there is a theoretical maximum. It just doesn’t make much sense to TV audiences since bigger number = more fast, in a linear way.

The warp scale did indeed change. I Actually seem to remember that was adressed in TNG.

Also the stardate naming convention changed after TOS.