Any further traditional contributions will be withdrawn on top of that amount and thus taxed at the applicable marginal rate.
You aren't the first person to be misled by the "marginal vs. effective" myth, and probably won't be the last....
I guess that's a reasonable way to look at it - have you filled the lower tax brackets with traditional yet - once you do, you might as well contribute Roth.
Yes. Pension, social security, interest, dividends, inherited IRA distributions, etc., can also add to the base income on top of which withdrawals based on future contributions will be taxed.
Of course if you expect to spend less per year in retirement than your taxable income in accumulation, you never get to the point where you expect your current marginal rate to be lower than your retirement marginal rate
In that case, you can estimate the marginal rates your heirs will pay. If your heirs are all charities, that will be 0%. If your heirs are your children who may still be working when forced to take inherited IRA distributions, it may be a high rate.
so the simplification works well enough to make a good decision.
Doing the right thing for the wrong reason? ;)
While traditional is better for most, we might as well give people correct advice on how to evaluate their own situations.