Lots of things are profitable but immoral. People will do crazy immoral and illegal stuff for money, but we outlaw and slander the more abusive stuff, like monopolies and such.
If it wasn't pensions that were funding PE, I'm sure PEs would get a lot more criticism and would not be allowed to do what they do.
Slavery is bad economics. If you want your economy to grow, paying workers is not bad. Economic growth isn't zero sum.
People who argue otherwise simply cannot escape their zero-sum worldview.
Yes you can pick up cotton at a lower labor cost with slaves, but you can't charge the slaves rent, sell them useless clothes, tax them, etc.
No but there would be a lot less PE dry powder available. To the parent's point - there is currently a trillion dollars in dry powder that is allocated to acquiring businesses. If that trillion dollars drys up then less businesses get acquired - it's that simple.