To determine the optimal extent to which the company's capital structure should rely on hybrid bonds, we can evaluate the standard trade-off theory of capital structure. 

In a theoretical framework to minimize the Weighted Average Cost of Capital (WACC), a company balances the tax advantages of debt against the expected costs of financial distress. 
1. **Cost of Debt vs. Hybrid Bonds**: Straight debt is typically the cheapest source of financing because it is senior in the capital structure and its interest payments are fully tax-deductible. Hybrid bonds (such as the "Titres Super Subordonnes" or deeply subordinated securities Veolia holds) carry higher yields than senior debt to compensate investors for higher risks (subordination and possible coupon deferrals).
2. **Tax Shields**: Hybrid bonds often receive only partial equity treatment by rating agencies and may not offer the same optimal tax shield benefits per unit of distress risk as vanilla debt.
3. **Replicability**: In standard corporate finance theory, any intermediate risk-return profile offered by a hybrid bond can generally be replicated more efficiently by a linear combination of pure straight debt and pure equity.

Therefore, unless there are rigid, non-fundamental rating agency constraints pushing a company to protect its credit rating while still borrowing, an optimal, strictly value-maximizing capital structure minimizes WACC by using purely straight debt (up to the optimal leverage ratio) and equity. Consequently, the mathematically optimal reliance on hybrid bonds in a standard cost-of-capital optimization model is zero.

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