That is correct, but is based on a philosophy that the longer something has been in use, the less chance it has to demonstrate behavior that has not been previously observed — he even explains that in more detail in further posts where he discusses his desire to not cause regressions.

Linus knows that and says so as well. But that's okay, because that works. Starting down that very same path again without a huge reason to do so would be very painful for any mature product.

Yep, that's how the evolution works. Understanding stuff is how the progress works.

Doesn't that mean that while it's actually happening, it's not, in fact, an acceptable development process?