This is a good response. In theory, if I put $N into an equity, then someone else will get $N out, because efficient market and all that. That frees the same

This is a good response. In theory, if I put $N into an equity, then someone else will get $N out, because efficient market and all that. That frees the same $N up for that other person to consume/invest. In practice there is somewhat the detail of equities being bid up a bit by people wanting to own them.