We are talking about people who "have a lot of money in retirement funds". A reasonable corollary is that they also have a significant other income or income-ish assets. Pension, stocks & bonds in taxable non-retirement accounts, etc.
Oh, by definition they do. Because they live quite well without Social Security, being as they are delaying taking SS.

I agree with all that.

That's a pretty big income range where they are solidly paying ~23% marginal tax. They pay the same rate whether their income is $100,000 or $400,000. So there is basically no tax reason/advantage to take steps to reduce their taxable income as long as it is in this range.

There actually are though. Let me illustrate with a real world-ish example. Let's say a couple retires early at 62. They can live comfortably on $150,000/year that comes from the sources you mentioned above, and an IRA worth $3 million at the start of retirement which they don't need for now.

That puts them in the 22% tax bracket, so they can safely convert about $50K/year before hitting the 24% bracket. At age 70, SS kicks in and they lose headroom to make conversions, but the account continues to grow at 7% until RMD's kick in. If I did the math correctly, at 73 IRA balance is $4.9 million, with a Roth balance of $590,000. The RMD on the IRA is $186,000 which covers all their spending, with just a bit extra.

Similar couple decides not to convert, and the IRA grows to $6.3 million in the same period. RMD is $237,735.85. That lands them the 24% bracket, blows through the first IRMMA cliff and maybe hits the second depending other income.

You're right if you look at just the difference in tax rates, that's not a ton of money. But the key point is they are paying taxes on money they didn't need or want. In this case it was roughly $9,100 bucks in more in taxes (not counting IRMMA) than the couple with the $186K RMD. That's enough cabbage to make it worth it.

And that's only part of it. The couple who converted has the Roth which compounds tax free for either them or their heirs. And they withdraw the money on their own schedule, there are no RMDs.

The only reason I can think of to not do Roth conversions is ACA subsidies. Everything else is pretty manageable.