While getting new stories published by traditional publishers in any format might be harder than it was, the internet has created the opportunity for any one to publish their own ebook, because no money is needed besides a computer and an internet connection. This has created a much bigger pool of authors to choose from. Not all of them write science fiction but enough do.
The market for used books has exploded thanks to the internet. It used to be hard to find already published books, you went from store to store. Books sat on shelves, authors went unread. The internet used book market has brought back to life countless numbers of books and authors, and keeps bringing more to the table. The ebooks still don't have a used market yet, though there is always the free route. The internet has destroyed local bookstores but in their place, global distribution puts far more eyes on books than a local store could.
The supply of used books is dependent on whether the people who end up with them, want to resell, keep, or throw them away. There is now a group of sellers who are book flippers. They cringe when they see a used book selling for $4, that's the old 1 cent for the book, 3.99 for shipping trick. You make a little money on the shipping. The book flippers go in and out of the market depending on what's going on. They constantly predict the end of $4 dollar books, and long for anything that returns a good profit. With a smart phone and an optical scanner app they are rescuing books, because a good number of used books are thrown away every year. There a organizations that look for books about to be thrown away and ship them to places in the world where there is a shortage of books, and sometimes no internet. This gives used books a new lease on life, getting to people who would never normally see a good selection of books.
Then we have the translation market which I believe is steadily growing in size and selection. This opens up both new stories and old stories to the market place. Technology can cut through a lot of the labor it used to take to look for, secure, translate and remarket science fiction, as well as other subjects, from a vast reservoir that goes back a hundred years.
Even if the percentages go down, and they do wobble a lot, serving 8 billion people means that even with a decreased percentage, there can still be more people reading or writing science fiction than there was with the higher percentage number. Formerly small niche markets can now have physically large numbers of readers and still barely show up as a blip on the horizon.
