I believe the efficacy of convalescent plasma has been shown to be minimal to non-existent, so I agree it isn't likely worth harvesting. If it were effective though, it would absolutely be worth it, because the United States isn't the whole world. There won't be enough vaccines to fully vaccinate the global population for many months at the least - probably at least a year. If we had an effective treatment that could be donated to countries not fortunate to have an abundance of vaccine doses, it would clearly be the moral thing to do.