Maxperson
Morkus from Orkus
The ones most likely to buy the new books are the ones who want them because they are 5e AND who have enough spendable money that the books don't represent a large percentage of their spendable income. The older crowd.As has been shown, all of thise age brackets are capable of buying every book WotC releases every year. Who is more likely to do so, though? People who already own years worth of books, or people to whom it is all new? People buying diapers and paying a mortgage, or enthusiastic youngsters with zero obligations?
This is just a reality if the market for a game ained at yourh, not sure why you are so adamant about this.
What you seem to want to ignore is that the vast majority of books released by WotC are monster books, which the older folks don't have and would need, adventures like Hoard of the Dragon queen which the older folks don't have, books like Xanathar's which have crunch that the older folk don't have, and finally new settings like Theros and the MtG settings which the older folks don't have. But yes, the settings like Spelljammer and Eberron which many(but not all) older players have would be something that a lot of them would choose not to buy.
There are no "years worth of books" that cover the new stuff.