Slavery is profitable. People only stopped doing it when it was perceived as immoral.
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.
The idea that slavery is a free lunch that is only banned on moral grounds is wrong.
Slavery is bad economics. If you want your economy to grow, paying workers is not bad. Economic growth isn't zero sum.