But you could have that upside by owning the collateral.
If I own the collateral, I own the upside and downside of the collateral. If I own a stablecoin pegged to the collateral, I own the upside and downside of the collateral, plus the risk of the stablecoin collapsing.
The stablecoin doesn't offer me any upside compensating for the risk, so we are left arguing that the risk of collapse is negligible, or arguing that there is some other benefit of owning the stablecoin to compensate for the additional risk of owning stablecoins instead of owning the collateral.
If I own the collateral, I own the upside and downside of the collateral. If I own a stablecoin pegged to the collateral, I own the upside and downside of the collateral, plus the risk of the stablecoin collapsing.
The stablecoin doesn't offer me any upside compensating for the risk, so we are left arguing that the risk of collapse is negligible, or arguing that there is some other benefit of owning the stablecoin to compensate for the additional risk of owning stablecoins instead of owning the collateral.