Both the 401k and MBCBP will be rolled into an IRA at retirement, so at that point my assumption is that all funds are co-mingled and withdrawals are both 401k and MBCBP.

Lots of wild assumptions going on here, including Trip
who has convinced himself he will make 20% per year in perpetuity, hedge zero percent of his 401k in lower risk assets, and end up with full income replacement in retirement after investing his taxable spill cash in taxable accounts, spend none of it on a Miata, and finally assume that 401k/Roth IRA rules remain constant but also assume tax rates will go up as a matter of course. If all that’s true, then he just might be right.