The data really isn’t clear that teachers’ unions have a causal benefit for students. In the 16 US States without teachers’ unions, when controlling for demographics, the data is mixed. Some of the positive systems that teachers’ unions tend to bring to the schools (peer review, etc.) have not been controlled for.
Taxpayer bills seem to inevitably be higher. This is a negative for the taxpayer if there’s not a clear demonstrable benefit. What is clear is that the model is incompatible with a free society (even FDR said as much), and there are no circuit-breakers to stop an out-of-control union in the public sector as there are in the private sector (the monopoly problem). State laws that remove incentives for unions to bargain only further tip the scales.
Reforming the corrupt system is the better option. Unions can work well in the private sector. In cities where inner-city youth have been given the option to take vouchers to private schools, all have flourished. An ecosystem of competing schools would not be incompatible with teachers’ unions, as the ones that negotiate bad contracts would simply be unable to provide an education for the voucher amount (or voucher+voluntary contribution). In many respects, blaming the teachers’ union for the problems of a State-enforced monopoly is scapegoating.