It’s incorrect that proprietary trading has negative externalities. It’s also exhausting trying to explain that to an audience that has fundamental misconceptions about economics, financial systems, and monetary systems, so I don’t think I have the energy today. Hopefully someone else will and add to this thread.
I never said it has negative externalities (whether it does or doesn't is a totally separate discussion), only that its positive externalities are certainly much less than some other fields that employ commensurately smart people, e.g. healthcare or climate change research.