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).

You seem to be using many different assumptions separately. In the first you assume you are maxing a Roth IRA (in my initial response I was also considering 401ks as many of them have Roth options nowadays). If you are maxing your Roth 401k and Roth IRA you are likely a high earner and therefore likely in a higher tax bracket than you will be in retirement. This means that kind of person will likely prefer traditional investments.

Your assumption there is someone maxing out their retirement options and in a relatively low tax bracket doesn’t seem like reality. So in your math example they wouldn’t be putting the extra in a taxable brokerage account but in the same tax advantaged account.

Quick edit: also I’m confused on the extra $400/year into taxable account. It should be $1,250 per year (25% of the 5,000) which would be closer to $600,000 before the capital gains tax.

I largely agree with all the points made here however I think the overall message is a bit misleading. I would disagree that Roth investments are the preferred for long term investments. You aren’t accounting for the opportunity cost of the taxes paid in the initial investment year. Those taxes, while small compared to what you will withdraw tax free are also losing out on 8x-ing themselves (as you would have invested that amount in a traditional tax advantaged account).

What this means is Roth is the preferable savings method if you are in a lower marginal tax rate than you expect to be in retirement. However traditional is better if you are in a higher marginal rate than you expect to be in retirement. If the marginal tax rate was the same when you invest and retire then the difference between Roth and traditional would be nil.