If students cannot be relied upon to cough up the cash, textbook publishers will start looking to raise more money from instructors and universities. Right now textbooks are paid for by students, but marketed to instructors, who choose the required text for their courses. Companies compete by providing instructors with complementary copies of textbooks, powerpoint slides, test banks, study guides, solution manuals, and so on. If the student market collapses, companies may decide to start charging for all of those beautifully prepared powerpoints.It's been a while since my college teaching days but back in the day, I got a chance to view this issue both from textbook committees and as a producer of some of the supplements Woolley discusses.
I've always held the minority opinion that students can and should actually read math textbooks and that one of the impediments to getting them to read the books is the fact that most of them are virtually unreadable. I lobbied hard for better written texts but I don't believe I ever convinced any other committee members of my point. As far as I can remember, the rest only looked for two things, the homework problems in each section and what order the topics were presented in. Obviously the committee had to make a decision and when most committees don't consider factors like clarity of explanation and price, that leaves lots of ties.
In this situation, assuming you do not allow kickbacks, very small factors often in up decisive. Case in point, video series as course supplements. Back when I was in the field, most books came with a video supplement, usually consisting of one of the authors working through a few examples for each section.
Back in the late 90s, I produced one of these for a college algebra textbook. I am not going to give you the name because it was one of the most God awful videos you'll ever seen, however since it did look more or less professionally produced and since it was long enough to cover most of the chapters and mainly since I got it in on time, everybody involved was reasonably happy and I got to pay off my car loan.
The sad truth was that most of these videos were pretty much useless. Most of the time they were put in tutoring centers where occasionally a student would try one or two and find they were not at all helpful. The problem was a combination of poor quality and the failure to think through exactly how video instruction should work ( insert MOOC comment here).
The reason these videos kept getting made was that they served as a check mark, a tie breaker. Committee members when faced with 10 or 15 books all of which seems more or less the same would look for some small but clear advantage that they could use to put some books at the top of the pile. These included videos that few students would watch, computer software that few would use and on the other side things like power points and lesson plans and test banks which the committee members probably wouldn't use but which they thought someone else might.
The thing about these tiebreakers is that they only have value when bundled with the product being sold and when the difference between the product and its competitors is very small. They serve much the same function as a $.10 toy in a three dollar box of cereal.
Historically the pay off for having a general ed textbook accepted by a major university has been huge. That was in large part because there was no practical option (legal or otherwise) for students who wanted to get a degree. If they couldn't find a used book, they simply had to pay what the publishers told them to.
There is no such monopoly on supplemental material, at least no effective monopoly. More importantly these supplementals are by nature non-essential. In fact many teachers, myself included, chose not to take advantage of them even when they were given to us for free. I preferred to make my own tests and presentations, and I know many teachers who feel the same way. That combination of monopoly, compelled-purchase, and principal agent problem which made the situation so dangerous with textbooks simply does not apply here.
We don't need to worry about text book publishers gouging instructors for free power points and test banks because these things are easy to do without.
Unfortunately, we still do need to worry about the rest of Wooley's concerns.