Thanks! It still leaves me with questions.
(Note, I know some of this seems to contradict your statement as to being unheard of, but I'm not sure the specifics of your case or what products, here me out, please.)
Is it reasonable to expect new products to have the same sales as the original/core products? i.e. Do we really expect a character options book (such as Tasha's etc) to sell as many as the core PHB? Or the twentieth adventure to sell as many as the first adventure? I don't think that is reasonable. The first books (of each type) should be expected to sell more as they are designed to appeal to the core demographic, while subsequent books (and adventures) are looking to fill in the gaps. Such as LMOP is a solid adventure. Just about everyone can use this at some point in there gaming 'career', but Strixhaven or spelljammer? No, they are only going to be of interest to a subset. There is still money in that subset, but not as much as for a core product.
Is that a valid intepretation of the data?
Note, imo it's not only about duration, but size of market and the money involved. Computers are not any older, but we have much better sales models for things like PCs, mobile phones, and all the software they use.