I believe this is valid, but maybe not widely practical.
If you retire in the year you turn 55, many 401k plans allow penalty-free distributions.
So let's say you plan to retire even earlier. You need to be an uber-saver to do that, so you are probably doing more than just maxing out a 401k and IRA every year. Either you've got a taxable account to tap, or a 457 (no withdrawal penalty).
OK, so let's say you're an uber-saver, but don't have a significant taxable account or 457 or Roth.
Roth converting during your last few working years could be a mistake -- these are often the peak earning years and peak tax rate years. (I suppose if the tax rate difference is less than 10% this could be considered better than a 10% penalty withdrawal, but surely an SEPP setup would be a better deal.)
Might work ok if you have a phased retirement--a few years of part time work.
But most likely you would wait until retirement before doing any Roth conversions, so that still leaves a 5 year window that you would need to fill. As you say, that could work if you have a pre-existing Roth.