In Lean FIRE, the early retirement years typically have the highest corpus-to-expense ratio (portfolio is large, spending is low). So 5 years of expenses may be only 10–20% of the portfolio, allowing 80–90% in equity.

This is exactly what Lean FIRE needs: maximum long-term growth with minimal drag from debt. At the same time, the safety bucket prevents the retiree from selling equity during a crash, which is the real killer in early retirement.

A fixed asset allocation approach misses this completely. It assigns an arbitrary debt percentage simply because the retiree is "retired," even when the actual spending requirement does not justify such a high debt allocation. Bucketing matches the portfolio to cashflow reality.