You run out of levels in a class, you enter a new class, simple enough. The career-level based benefits table is easily extendable.
I have to agree with this line of thinking, there is also a bit of "truth" to it, there comes a point of diminishing returns with skill vs time doing said skill. I see it at work the guy that has been doing it for 20 years is not necessarily any better then the guy that has been doing it for say 15 or even 10 years.
So branching out into new fields makes sense especially when you have expert and master classes to chew threw.