Playing a champion well = winning.
Having a good score doesn't necessary mean you played well either.
I'm not contradicting myself... I'm not saying the system is perfect, you can play well and not get a lot of points, that's true, but if you deserve to get mastery fast then you'll win more than you lose.
The only near example I can think of is ranked.
If you truly play well you'll climb fast and lose some like everyone.
If you don't truly play well you'll win some and lose most.
It's impossible to balance this with an algorithm, as I said, the way it's set up is the best way to balance the pros and cons...
To be completely honest I made a post near identical to yours a few months back but then I realized it's just not that easy to balance...
So TLDR if you've gotten good with the champion you'll still win more than you lose getting mastery fast but if you haven't then you'll get mastery slower...
It's a complicated system but once you understand it you realize there's not really that many other ways to go.