"But, if the production as a whole, taking into consideration whatever income was generated from Netflix, ends up returning its investment, isn't that a win?"
First of all, I doubt it would but if we assume your "if," no I do not think so. Second, Diana has not opened on Broadway so it is of course a lousy example even of what you are suggesting. But in general, no I would not say that is a win. People produce theatre (in my opinion and also in my observation) because they have a deep-seated belief that live performance is the coolest thing on earth. These filmed things are not and, frankly, I don't really have much of an appetite for them especially when we are not cooped up at home. I think they are pale imitations. Even Hamilton. (Maybe especially Hamilton.) Just returning an investment is not the "win" of making theatre; it is a goal but to me what you are proposing is missing the forest for the trees. And to me the logical extension of what you are proposing is also that we would reach a point where we say (in my best Bette Midler voice) why bother (opening on Broadway at all).