My point is that this what happens in real life. People will continue to engage in stuff that they want to engage in. Facebook doesn't force people to engage in anything.
I think it is far worse to attempt to condition people by showing them things *they wouldn't otherwise engage with". What is scarier, showing somebody something they want to engage in based on their past behaviour, or showing somebody something they wouldn't have otherwise seen if you hadn't gone out of your way to shove it down their throat?
People seem to want Facebook to make people more placid. Oh you have extreme views? Here's, let's condition that out of you by only showing you more moderate stuff. Oh you think x is bad? Let's not show you anything to do with x so that you'll hopefully forget about it and not engage with that part of your brain any more.
Like I've already said, this alternative is far more Orwellian and far more of a tool for social control, than simply optimising for engagement.