What you fail to comprehend is that over the years cruise lines learnt how many passengers prefer club dining and how many freedom dining, and they set their dining rooms up to offer that split. Hopefully the cruise lines got it right more often than not.
But of course any average does have seasons which dont follow the normal trend, and over time it has seemed that an increasing percentage of passengers have shown a preference for freedom dining, which presumably is why P&O have opted for all freedom dining MDRs on Iona and Arvia. But you are correct that this does benefit saver fare passengers, and unless the extra OBC for select fares is high enough, some usual select fare passengers will undoubtedly decide to try a saver fare, until they end up in an unsatisfactory cabin.
I suspect that Aurora and Arcadia will totally drop the app system fairly soon, and maybe Ventura as well. But the demographic on Azura for fly cruises, and the tight MDR dining capacity on Britannia, will probably mean they will try to tighten up on the app usage.