To determine the optimal extent to which the company's capital structure should rely on hybrid bonds, we evaluate the cost of different sources of capital. 

The weighted average cost of capital (WACC) minimizes when the cheapest sources of financing are prioritized. 
Looking at the market data for 2022:
- The cost of senior debt (proxied by the iShares Core Euro Corp Bond) has an average yield of 1.085%.
- The cost of subordinated/hybrid debt (proxied by the Sub-Sen Delta) is inherently higher (indicated by the 2.295% average for the Sub-Sen Delta). 
- Cost of equity is the most expensive, typically calculated using the risk-free rate (1.927% for the 10Y Swap) plus an equity risk premium adjusted for beta.

Because both senior debt and hybrid bonds are generally tax-deductible, we evaluate their after-tax costs. Since the yield on senior debt (1.085%) is strictly lower than the yield on hybrid debt (1.085% + spread/delta or simply the 2.295% sub-sen yield), the after-tax cost of senior debt will always be lower than the after-tax cost of hybrid bonds. 

In a pure WACC optimization without qualitative rating agency constraints (where agencies may treat hybrid bonds partially as equity), a company will minimize its capital costs by issuing 100% senior debt instead of hybrid bonds or equity, as senior debt is strictly cheaper. Because hybrid debt is consistently dominated by the cheaper senior debt for standard debt financing, the theoretical optimal reliance on hybrid bonds to minimize WACC is zero.

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