Champion Mastery and Winning Games

So, let's talk champion mastery.

Right now, for a champion to be "mastered" it will take 30+ games, assuming a 50% win rate. That ain't so bad, especially for people like me, who don't play ranked. This puts forth an interesting goal to strive for on League. Akin to many console games, LoL can now be 100%'d. It's an awesome thing. And even when one hits lvl 5 mastery, they still can rack up points, so there is still a clear difference between the true masters and the people who have grinded games. That should be satisfactory for all the single-champion mains out there.

However, there is one component of champion mastery that really rustles my jimmies. That is how dependent champion mastery is on wins Now, don't get me wrong, winning games should definitely give more points than losing one, it's just common sense. However, as it stands, right now, champion mastery is almost completely based on winning each game. For instance, I played a game as Jax. It was a good game, I ended up 23/10, but ultimately we lost. Checking my score in the end game lobby, I had earned an S+ for that game, a non to shabby feat. However, even though I am only a level 1 Jax, I only filled 1/8 of the champion mastery bar. On the flip side, I've had anywhere from a B+ to an S- victory fill over half the bar.

Why is mastery so based on winning games, instead of actually preforming well with the champion?

This concludes my rant. What's your stance on mastery?