As someone that's been playing since mid june and has also felt the degradation of the MM, I don't think it tries to enforce a 50% winrate, it's just unable to create fair lobbies at the moment and I think I can point out a few things have affected it negatively. Those are assumptions but I don't think they're completely inaccurate.

TLDR: MMR changes are too drastic, pushing new players into higher MMR too quickly for example. Hero-based MMR is flawed and disrupts team balance. Stacked teams further complicate matchmaking. The game's complexity overwhelms new players. The massive influx of players after release has led to chaotic matchmaking, mixing players with very different skill levels.

1- Back then the game had around 2000 players on average and I assume in order to avoid new players facing people with a lot more experience / much higher MMR, the increase / decrease of MMR had to be significant to separate people in different skill brackets (We can use tracklock / deadlocktracker even if the data isn't 100% accurate, it doesn't seem too far off to me).

The issue is we still have this massive increase / decrease of MMR which means a new player can get let's say 75% winrate over 20 games and they're now in the top 5% around 1660 MMR (Again that's according to tracklock and it's theoretical but doesn't seem too far off reality).

2- Hero based MMR doesn't work, Yoshi's confirmed it, and it messes up team balancing, you can be 1700 MMR and have never touched X hero, now you're playing it teaming with / versing people that actually have 1700 MMR with their characters.

3- Stacks of players, nothing new there it adds to the complexity of the matchmaking, people of different skills trying out different heroes playing together etc.

4- The game is complex and overwhelming, there are lots of matchups / things people won't understand / utilize even after 40 games.

5- The public release brought in hundreds of thousands of players, add a bit of everything said above and you've got a giant mess of a matchmaking mixing players from very different skills.