My take on this is that while we should definitely have done better, big clubs aren't primarily concerned with player profit. They're concerned with what they're getting on the pitch. That's why we didn't sell Aubameyang at peak value in the summer of 2019 for instance, which I'm sure is what a small club would have done in a similar situation.
Our biggest failure has been that we've lumped so much money on players who gave so little back on the field. Just look at Mustafi, Mkhi, Xhaka, Pepe, Willian, and so forth. Have they had ten really good games between them? When people say that we need to spend money better, this is what they mean. It's not about squeezing a couple of million out of endstage Mustafi; it's about signing players that we don't want to sell again.