You use defunding as if it is a bad thing. The purpose of the public school system is not to maximize funding or revenue.
School choice/vouchers defund public schools proportional to loss of public school responsibility for students.
School choice/vouchers defund public schools proportional to loss of public school responsibility for students.
Besides, it is pretty said that the best counter argument against charter schools is that parents will take their fair share of funding and use it to buy a much better education for their children.
To your point about charter schools, there is a strong argument to be made that when parents have a choice where there kids go, public schools will have to make sure they are providing more academic opportunities if they don't want to risk losing the academically minded families (and their money). That seems like it would be a good thing for everyone.