Did I say mandatory? I meant optional! You’re “free” to die in a cardboard box under a freeway as a market capitalist scarecrow warning to the other ants so they keep showing up to make us more!
If you are maxing your Roth 401k and Roth IRA
You can’t max both, you have to choose between them.
Your assumption there is someone maxing out their retirement options
The math doesn’t change if your contributions are smaller. This is a game of percentages.
You definitely can max both a 401k and IRA you can also max an HSA. All these combined allow for an individual (not family) to save $34,150 (there are some edge cases I’m ignoring). Someone who can afford to save almost the US median salary amount is almost certainly going to have a higher benefit from traditional than Roth.
Also, the math completely changes if you aren’t maxing because if you aren’t maxing the contribution than adding more to the contribution gives you more tax advantaged investments (instead of taxable brokerage investments).