Mandatory Roth catch-up rules taking effect in 2026 will eliminate the upfront tax deduction for high earners, fundamentally altering the math for those with large traditional 401(k) balances. For a 58-year-old in the 32% bracket, the loss of the deduction on an $8,000 catch-up contribution amounts to roughly $2,560. A $2.3 million 401(k) growing at 6% will produce six-figure required minimum distributions at age 73, which when stacked with Social Security can trigger IRMAA surcharges and push effective tax rates to around 40%. By contrast, long-term capital gains and qualified dividends in a taxable brokerage account are capped at 23.8%, a gap that could save $64,000 on $400,000 of retirement withdrawals. The analysis recommends contributing only enough to capture the employer match and redirecting the freed cash flow into a taxable brokerage, while using the years before RMDs begin for Roth conversions that stay below the first IRMAA threshold.