Why not pose the same question regarding virtually every other book being released by a major publishing company these days? It doesn't appear to be hurting sales of books directly.
But the point is, Simon & Schuster would have to spend extra money
on hiring a production team, musicians and actor to record the unabridged audio (many, many hours of work) for perhaps little profit, since you are arguing that fans who hear an unabridged novel don't even then go and buy/read the hardcopy novel.
The abridged audios were an advertisement for the longer novel, which fans willingly paid for, or/and
a unique chance to hear a brand new vocal performance by favourite ST actors: Nimoy and Takei or Doohan, in the early days. Later Frakes, Besch, Koenig, McFadden, Dorn, and others.
I really don't care for unabridged audios; I have no need for them. I have nothing against S&S making them, but if you want to prove a case for them, you need to convince us/them just how they'll be economical and/or profitable to create/sell.
As for the clumsy public library packaging for unabridged "Sarek", the problem was the need to package ten
cassettes. This problem has been eliminated due to CD technology and digital downloads.