We're not talking about a binary choice. Rather, low Treasury rates influence the risk-interest demand curve across the broad market. People are pushed into low

We're not talking about a binary choice. Rather, low Treasury rates influence the risk-interest demand curve across the broad market. People are pushed into low yield on low risk investments, which pushes yields lower on marginally more risky investments, &c. Furthermore, leverage allows you to make portfolios that mimic the risk of other portfolios of riskier/less risky assets. In a theoretically perfect market where the replication can be exact, this fact alone can generate the entire risk curve.