When you're retired and not working, drawing down from a Traditional 401k is taxable income. You start at zero and go up from there. ...
How is this incorrect?
It's incorrect because not only pensions, SS, etc., but also withdrawals based on previous years' traditional contributions fill those bottom brackets.
E.g., if one has been contributing to traditional accounts for 20 years, the withdrawals based on the 21st year of contributions won't "start at zero" - correct?
No idea, actually.
Ok...in that case are you willing to believe the Bogleheads wiki I've used for several of the links on this?
Again, this is a retiring early forum and the thread was posted by someone in their 20's. I am not thinking they'll be contributing to a traditional retirement plan the year before being eligible to make withdrawals from said plan at 59 1/2 years old. Most people will have a 5 year gap before they can structure a conversion ladder, yes?
And for someone just starting*, traditional is likely the better choice. But for others reading this (say, those in their 30s, 40s, etc. - or to benefit those 20-somethings in years to come), giving correct advice as generically as practical is worthwhile. In this case, "when (if ever) to switch from traditional to Roth?" depends (among other things) on how much one has already contributed to traditional accounts. And the way to evaluate "how much is too much?" is to compare marginal to marginal.
See the full Traditional versus Roth - Bogleheads wiki, and the background (links and footnotes) to item #4 in Investment Order for more details. Does going through those make this clearer, or is it still muddy?
*Med school grads in residency are a notable exception due to their much higher expected incomes in future years.
If we're talking about retiring at an age where you're eligible to withdraw immediately from a Traditional 401k and you're also receiving social security, I'm out of the discussion. That's not my game, and I'll be the first to admit I don't know shit about it.
Nope, SS is just one of many things that can push traditional withdrawals into higher marginal rates. We can eliminate SS and pensions from the discussion entirely, and still be left with marginal vs. marginal.
I have done literally no research on retiring at 59yo+ and have no opinion on the matter. You can spread whatever information you want, though I find it largely irrelevant to the OP.
The question wasn't "what generic advice would a 40-something want" -- it was quite specific, with an age range and income bracket provided. I recall a few links and a "those guys are wrong, despite the sources they cited, here are some other sources and why Roth might be better"...despite Traditional likely being the superior choice, which you just said yourself (and is also listed as the safer choice in the links you just posted). I must be missing something, unless we're supposed to assume that everybody posting here, regardless of circumstances outlined in their post, are going to be retiring at social security age, collecting a pension, and contributing to a traditional IRA up until the year before they start withdrawing from it. Valid points, to be sure - but again, with a 20-something posting on an ER board? I'm not sure how well that fits.
The bogleheads wiki also agrees:
Tax considerations:
If your current marginal tax rate is 15% or less, prefer a Roth.[note 1]
If you expect to have higher marginal rates than your current marginal rate for most of your career, prefer a Roth.
If you will have a traditional account or a pension large enough to meet your expected retirement expenses (and you expect to take that pension shortly after retiring), prefer a Roth.[3]
Otherwise, prefer a traditional account.
Unless the OP has enough money to retire today, at least. I disagree with their second point; if my marginal rate is 25% now and will likely be 28% later, I will still contribute to a traditional 401k. Career marginal rate is irrelevant, as you noted earlier. What matters is rate at contribution and rate at withdrawal.
If you're simply disputing the way in which my post was phrased, and insist that it's wrong because it's not universally applicable to everybody's retirement situation - then sure, I agree. It's not.