In some ways this is inevitable, though. C2, Proficiency, or whatever label we attach to that 'level' is fundamentally different from the previous stops along the journey, as it tends to be ascribed the meaning of 'you can now take the controls and fly the plane yourself'.

'You can be professionally effective without support, or can genuinely benefit from a level of advanced study in a subject almost as if you were a native, without frustrating yourself or others.'

The demands step up more significantly not just because of what you can do with the language itself, which is a graded continuum, but because you are expected by others and yourself to be able to do so almost on a par with native speakers, and with hardly any allowances being made for the fact that in fact you aren't.

The dual control from teachers or colleagues is deactivated and you're going solo.