Here is how I made the decision to send DSs (age 8 and 10) to private school - and no it is not an absolute necessity at this stage but will be later.
I have never owned a car and never owned a house. I can afford to own a car and a house but I deal with the necessity for shelter by renting a house and the necessity for transport I deal with by hiring a taxi or taking the bus. I save a lot of money but occassionally it is a bit inconvenient and not quite as luxurious as if I owned my own car and house.
I also have a necessity to educate my children. I could meet that necessity by sending them to the one local state primary school my DSs would be eligible for but it does not provide very good facilities and does not provide wrap around care. The quality of the teaching is good but there are 30 pupils per class.
If the state primary school had small class sizes, good facilites and wrap around care I would be sending my DSs there. Instead, I send my DSs to a private school which provides the best overall education I can afford in the place that I live. It has good facilities small classes (15 per class) and wrap around care. In that sense I choose to spend my money on something that I think is a good investment in my childrens future but it is not an absolute necessity.
Let me be clear, if I had a very good state Primary school near where I live I would send my DSs there, save the money and get a better education for DSs too. The private school my DSs go to is good but not outstanding. It is just better than the state Primary schools on the dimensions that matter to me. I went to a good state Primary school myself.
At secondary school level the gap between state and private school becomes a gulf in the area we live. At that point, I would regard private schooling as a necessity because the state school simply fails to provide a good education in the opinion of Ofsted and parents I have spoken to. I could get a better state secondary education for my DSs by moving house and paying for a tutor (an absolute necessity) to get them through the Grammar school entry exams in the LEA next to the one we live in.
I don't think wanting a good level of education for DSs is wrong and I am sorry that some people have no choice but to send their DCs to bad schools. I do not think that is my fault though. A good education to a good minimum standard is a necessity and if the state cannot provide it in ther area some parents feel they have no choice but to pay for it.