dude, I have a few friends who were eligible for PSLF until they left for much higher paying private sector careers, though it did weigh into their thinking when they were leaving the public sector. The scenario I posted would certainly be rare, but would result in a higher achiever (in HS and career wise) receiving significantly lower overall compensation.
My solution wasn't to do away with it, but to allocate a set amount that can got to repaying student loans OR towards retirement if you make it to 10 years. Seems like that would be an even bigger win (though cost more taxpayer $) and further prevent the brain drain you describe.