To determine the appropriate extent to which Veolia Environnement's capital structure should rely on hybrid bonds, we must evaluate the company's financial profile, existing capital structure, and the strategic role of hybrid securities.

1. **Business Model & Risk Profile**: Veolia is a major utilities company in the water, waste, and energy sectors. It operates under long-term concessions and regulated contracts, providing highly stable and predictable cash flows. This low business risk allows the company to sustain a higher proportion of debt in its capital structure compared to cyclical industries. 

2. **Existing Capital Structure & Metrics**: Looking at the 2022 annual report data:
   - Total Equity (2023-01-01): €14,867 million
   - Total Financial Liabilities (Noncurrent + Current excluding concessions and leases, 2023-01-01): €19,692m + €6,521m = €26,213 million
   - Currently, hybrid bonds (Titres Super Subordonnés ADuree Indeterminee) stand at €3,496 million. 
   - The existing hybrid debt makes up roughly 19% of the total financial debt and about 24% of the total equity base. Thus, the company already relies on hybrid bonds for roughly a quarter of its equity cushion.

3. **Credit Rating & Subordination**: Veolia is an investment-grade company (typically around BBB/Baa1). For such companies, rating agencies (like S&P and Moody's) treat eligible hybrid bonds as 50% equity due to their perpetual nature, deferrable coupons, and deep subordination. Relying too heavily on hybrids (e.g., 50% or more of the equity stack) dilutes the true loss-absorption capacity of the firm because hybrid investors are deeply subordinated to senior debt holders. In a stress scenario, a capital structure with 50-100% hybrids would lack sufficient senior equity protection, leading to severe downgrades and violating debt covenants.

4. **Acquisition Financing**: The significant increase in hybrid bonds from €2.46b to €3.50b (and the "Contribution Of Hybrid Debt Suez SA" of €1.62b) was strategically used to finance the Suez acquisition while protecting the investment-grade credit rating. This confirms management's intent to use hybrids as a bridging instrument rather than a foundational layer.

5. **Market Context**: Given the rising interest rate environment in 2022 (evidenced by the swap curves shifting from negative to ~1.7-1.9%), the cost of issuing perpetual hybrids has increased. Over-reliance on expensive perpetual debt in a high-rate environment is suboptimal compared to traditional senior debt or equity.

**Conclusion**: While hybrids are an essential tool for Veolia—currently comprising roughly 25% of the equity base—relying on them beyond this proportion would undermine the firm's creditworthiness and subvert the senior debt holders' security. A 25% reliance perfectly aligns with the company's current practice, rating agency guidelines for BBB-rated utilities, and the structural need to maintain a robust senior equity layer.

<answer>25%</answer>