I think I disagree with both points you make above
Whilst no system is perfect, I think the current competency based system for L4 to L2B referees is in pretty good shape. It gives disproportionate weight to the three things that really matter to all concerned (AOL, Decision Making and Match Control). It is clear to both referee and Observer what is expected and the competencies are varied enough to account for a variety of different match situations. And certainly I feel like my abilities have been positively shaped by the input I've received going from L7 to L4 (and whilst at L4).
As for the 'frequency of mistakes not diminishing at more senior levels', you've got to bear in mind that the games at higher levels are faster, more intense and way more pressurised. Dealing with thousands of fans rather than tens of them is a whole new ball game. And throw in the knowledge that your actions will be captured and minutely dissected on video. Even if the frequency of mistakes doesn't decrease (which I'd challenge .. the mistakes are just more in the spotlight) then the officials are still way better as they are dealing with more difficult environments