Why should D&A and capex converge in the steady state (terminal year)?

In the steady state a company invests exactly as much as it depreciates — 'maintenance capex'. If capex is permanently > D&A: the company is still growing, but growth does not belong in the terminal phase (by definition a steady state). If capex is permanently < D&A: the company is shrinking (its asset base is being run down). In the terminal year the two should be roughly equal, or capex slightly higher to offset inflation and modest growth (capex ≈ D&A × (1+g)). Important for a clean terminal-value calculation.