In the previous post I questioned the validity of ‘number of journals’ as proof for the amount of publishing activity in open access. A follow-up question I have is this: why is all this a priori ‘proof’ necessary in the first place? What ‘proof’ is needed to show that open access articles are accessible to more people? It is in the very concept! What ‘proof’ is needed to demonstrate that paying an amount upfront for the service of publishing is worse, or better, for its economic sustainability than paying for subscriptions? The proof of that particular pudding is simply in de eating.
Any choices between open access and non-open access will be made by those who actually have the choice: authors and their (financial) backers. The latter (the backers) can even impose that choice. Publishers can't – and shouldn't. The only thing to do for publishers – be they societies or independent outfits – is to offer the choice.