That doesn't follow at all. I think you're projecting how you would have made the decision, not how WotC, based on what we've seen of their past behavior, made the decision.
All it takes is for someone senior inside WotC/Hasbro having the gut feeling that most of the a la carte folks would purchase entire products if that was all that was available and having the seniority to make it so.
except that they know how well the parts sell and how much effort it is to make them available separately…
Could they just have flipped a coin whether removing them results in WotC making overall more revenue or not, theoretically yes, I just do not think that is a feasible scenario
To me the actual scenario looks like this
1) WotC is aware that selling parts is not making a lot of money (and possibly causes a disproportionate amount of work relative to that)
2) they test the waters by not releasing BoMT in parts and get little to no negative reaction
3) they pull the plug on selling pieces individually
The existence of 2) pretty much ensures that it was not just some gut reaction, and don’t forget that they do have the data from 1), so making the case that they have it but ignore it in their decision making is a heavy lift
We know that WotC doesn't market-test their ideas or do research on some of their sweeping decisions, because less than 18 months ago, they got themselves into mainstream news around the world by trying to revoke the OGL,
Much easier to market test something and keep it under wraps when you own the marketplace, so I would not compare the two
which even if they'd talked to their lawyers about first, they would have known was impossible.
it ended up being market tested and they retreated
as to a lawyer telling you it is impossible / possible, you can find one for either position, plenty of posts from lawyers here ended up on both sides of this