It is a weird state business which gets most of it’s income from the state, you’re right. If we were to get all of the underlying numbers from OS and other trading funds, I wouldn’t be surprised to see that the private sector contribution is much smaller than what government puts in. As you disappear further and further down the rabbit whole trying to understand a) how much actually OS costs and who their customers are, and b) how to actually prove the broad economic impact of open data release, everything becomes more opaque and hand-wavy unfortunately.
Which is why in the end it all comes down to high level principles and political ideology. There are just some politicians who are so committed to a private sector provision of services — any service — that they are immune to a principled, risk-based argument that in some cases a state provided data agency could be run cheaper and more efficiently if it wasn’t being asked to make a profit and gave data out for free AND this would also produce benefits in terms of both public and private sector efficiency and innovation. For them, only efficiency can be realised in the private sector and there’s no room to conscience innovation happening within, and in response to, a fully funded state institution without a private sector element. It doesn’t compute, and would begin to question their wider project, which to them would be dangerous.
Again this is why you need a different kind of politics (and economics) where the positive benefits of sharing are made explicit. Where the quid pro quo is that government provides a universal service for free but there’s an expectation that those in the private sector who are benefitting pay their fair share in both collaborative engagement in the market and through a fair tax system.