Every champion should have a 50% win rate

thats statistically impossible for every champ to be at exactly 50%

In addition to that being insanely unlikely even if all the champions were perfectly balanced, some champions are balanced when their winrates are under 50% because of their learning curve. If players can play their first 6 games as Vayne and on average win 3 of them, something is wrong with Vayne. Some champions require a lot of time to learn properly and do well with. Those players new to her should bring down her winrate, which is why she is balanced when its under 50%.

That's not even possible when you factor in the skill levels of different players.

rock paper scissors is balanced and none of the 3 have a 50% winrate same with rock paper scissors lizard spock.

It's impossible.

Less popular champions, for example, will always have more than a 50% winrate because only mains play them.

More popular champions will always have less than 50% because less skilled people use them.