That's now how matchmaking works, it's how maths work. You can't have fair matches without winrates eventually becoming 50%.
You're not being pushed to 50%. The law of big number simply makes all prior wins negligible once your MMR reflects your skill.
Let me elaborate with an example:
Let's say you win 10 games straight without losing a single one. You miraculously end up with precisely your "true" MMR after this.
Winrate so far: 10/10 = 100%
Now that your MMR accurately reflects your skill, all of your matches are fair, which means they have a 50% chance to go either way. You play another 100 games; 50 wins, 50 losses (for accuracy I actually ran a program for coinflips... literally ended up 50/50).
Now your winrate is 60/110 = 54.5%
In this hypothetical scenario you were not actively pushed towards 50%, you simply played ten times more fair matches than the initial ones where you were (likely) the most skilled player in the game every single time.
Edit: More mathematically and general, for those who are interested
You play n games, with w wins before you reach your "true" MMR and thus fair matches. Then you play N more games.
After n games, your winrate is precisely w/n.
Of the N games after that, you are expected to win half (since they are fair matches).
So after the entire N+n games your winrate can be expected to be (w+N/2) / (n+N)
= w / (n+N) + (N/2) / (n+N)
= w / (n+N) + 1/2 * N / (n+N)
= w / (n+N) + 1/2 * (N + n - n) / (n+N)
= w / (n+N) - 1/2 * n / (n+N) + 1/2 * (N + n) / (N+n)
= (w - n/2) / (n+N) + 1/2
As N approaches infinity, the left term approaches zero (since the numerator w - n/2 is finite), leaving us with the limit of 1/2 = 50%.
It's inevitable. You will end up with about 50% winrate the longer you play, without anybody rigging the matches against you.
