I'll have to think some more about that.

Problem is, almost no papers and articles even touch on that subject.
Most of them just say "Paying the tax with outside money is a super clever way to stuff more money into a Roth." That is, they completely ignore the alternative uses for that money; they just assume that it magically appears.

There is a small amount of voices in the wilderness discussing about that there is no benefit to a conversion unless your marginal tax rate is smaller now than in the future.

I don't recall reading any article or paper that delved into tax that gains on the outside funds would be subject to.

My gut feeling is that this "obvious" take is incomplete and quite probably wrong.

Just at first level, it is the case that the money in an IRA will be taxed eventually. So why go through gyrations to pay the tax now rather than as far in the future as possible?

The two major cases I see are if your tax filing becomes single instead of joint, and you voluntarily pay the tax so that your heirs don't.