Yes, thank you for mentioning Plan9, it's a wonderful example of the limits on current OS research.

The arguments you give are spot on, I would only add the need for backwards compatibility, or at least the possibility of maintaining the huge and ever increasing volume of mission-critical systems running Unix (Banking systems, the financial world, the Internet, ...).

However, while it's true that Plan9 failed to catch on, a lot of its ideas have been backported to Unix (/proc anyone?). I think that there's something there. Unix can be improved. But no one is going to look at your improvements if it breaks their existing systems.