Exactly.

The example of X.org going modular is valuable here. There is a reason that XFree86 did not go modular, or make the other significant changes that X.org has. That reason is inertia, as you say. Developers are used to a continuous development process with rules and aspects of the project that just work the way they are. Discontinuity is the only reason that inertia got broken -- otherwise, any similar effort would have been a minor fork, and likely unsuccessful.

It's the difference between evolution in a static environment vs. under the conditions of punctuated equilibrium. While Sun is in control of Java (I don't expect that to change anytime soon), development will most likely be done in natural progression from the way it is being done now. It would take a punctuation of that equilibrium to bring about this sort of change; its desirability alone will not overcome project inertia.