Charter schools are public schools so saying they take money away from public schools is an oxymoron. By virtue of the fact that they are public they are not for profit. They are literally incorporated as nonprofit entities. These are basic facts that people still get wrong despite charters being around for 20 years.
But I realize that is not your point. Saying that money is taking away is repeating a line used by anticharter advocates. What is less biased is to say that when parents decide to send their children to nontraditional public schools, the money for those students follows them to their new school. Why should the money stay at the school where the student left? The dollars are for the students and thus when they attend a different school the state gives the money to that school.
Problem is that there is a barrier to entry, even in a perfect charter school, parents still have to take the initiative to research the school and then sign up their child. It further segregates the students and in this case the population that's the most at risk of failure.
I posted the John Oliver video to highlight how charter schools get around being non-profits. Most of these are not funded by millionaire celebrities and philanthropic funds. I highly doubt lobbyists backing Republicans are opening charter schools out of goodness of their hearts.
I'll take your word about 6 years being the ceiling for teacher improvement but just from my anecdotal experience I had friends writing curriculum two years into their careers and one becoming vice-principal 3 years into her teaching career at a charter.