You're conflating ranking and matchmaking systems.
A ranking system measures relative skill/performance levels. You can have a ranking system without using it for matchmaking.[0]
I don't agree that a typical 50% win rate[1] indicates a successful matchmaking system. For one thing, creating fair matches is _a_ purpose of a matchmaking system, but not necessarily the _sole_ purpose. For another, that people win half their games on average says nothing about how fair the matches were.
I think that fairness often gets prioritized over fun. Playing sports should be fun at all levels, but it's particularly important at the lower skill levels that the participants enjoy themselves. That's how sports grow and become cultural institutions. Being a low skill player in a silo of other low skill players is a decidedly un-fun experience that drives a lot of new players away from e-sports. A ranked matchmaking system could be designed with the express purpose of helping low skill players have fun and naturally develop into average skill players.[2] I wonder what such a system would look like.
Assuming there are no ties, and teams have an even number of players, a multiplayer competitive video game is going to be a zero sum game; for every winner, there is going to be a loser.
While I agree that you it isn't the SOLE purpose of a matchmaking system, I do think a fair matchmaking system will end up with most players having a 50% win rate (with a few people at either ends of the skill spectrum having lower or higher win rates). If you are winning more of your games, you should play better at better players until you start losing again (and vice versa). You should eventually hit an equilibrium where you are playing people you have about a 50% chance of beating.
most ranking put player in an oscillating loop around their equilibrium, often that 50% win rate is not because a 100% fun matches but because a sum of 50% unwinnable frustrating matches and 50% easy boring matches
measuring win rate does shit like that
and then most system double punish you for quitting the unwinnable matches
that metric is possibly the second most frustrating aspect of online play
I think ideally you wouldn't show the ranking to the player, just use it to create the match.
With a large population, everyone should end up winning about half their games. That would be the sign of a successful ranking system.