I would urge people to remember there is a dominio affect to school places.
If you decline a place at your state school in early June, that place will be offered out, but the admin around that is likely to take a week or two, whilst the family decides and completes the paperwork. Assuming they take the spot, then their previous place will be offered out, and again this will probably take a week or two, and so on. Multiple children will potentially miss transition days. Also schools often plan their secondary classes to put children with some others from their primary etc. Any last minute place will just have to slot in wherever is left.
If you don't release your place until early Sept, people will have incurred uniform costs, plus children will be unsettled by changing schools, especially as this domino process may take until Oct half term or longer to complete. There may even be families who have moved to the area with no space at all.
Whilst I understand the thinking (let's keep this place in case something unexpected happens between now and Sept), something unexpected could also happen in October, and you would have to respond to that without the state place backup.
I know you (OP) didn't say this, but it's incredibly selfish to hang on to a 'just in case' place for 6 months, that someone else really wants. It's decision time now, and if you are too anxious about the financial risk, then just take the state place.