I felt the whole purpose of the mastery system was to be able to show skill in a champion despite win/loss 'luck' from team performance in the short term.
It didn't do this at all.
It only matters if you grind, you need to play many games to get the points anyways and you're playing that many games you'll likely have a 50% win rate over that time. Since what matters most by a mile is if you win or lose, your mastery level is strictly dependent on playing a lot. Not at all dependent on actually doing well.
one should think:
Mastery is NOT lots of practice. Mastery is lots of GOOD practice.
Practice does not make perfect. Perfect practice makes perfect. Repeated mistakes make bad habits. You could play 1000 games as champion z, if you keep getting C ratings you probably shouldn't even be rank 2 with the champion.