The most frequent question that is asked in scholarly communication circles about gold open access is whether a business model is sustainable and/or scalable.
Assuming, for the sake of argument, that we are talking about publishing the exact same quantity of material as we are under a subscription model, here’s what that means:
Does the model distribute costs in a way that makes it affordable to the actors who pay? For instance, APCs might be unsustainable because they concentrate the burden on specific institutions rather than spreading it among many. If the model does not distribute costs, then it is not likely to be sustainable or scalable.
Does the model encourage free riders? In other words, is there a way in which, beyond doing things more cheaply, a gold open access model might take money out of circulation by encouraging people not to pay? If so, then there is a challenge for the remuneration of publisher labour (though excessive profit margins mitigate this “risk”) and it may not be sustainable for a publisher.
Does the model take money away from subscription expenditure/publishers (i.e. offsetting)? If not, then the model is not likely to be scalable since it is an additional cost for libraries to bear. Hybrid without offsetting does not fulfill this requirement.
Does the model alter the financial flows back to other incentivized entities, such as learned societies? Argue all you like that this is a bad way of funding societies (it is) but if a model disrupts the revenue of learned societies, they will see this as unsustainable.