I'm not really understanding that. If you convert IRA to Roth at a 15% tax rate, to avoid later RMD at a 25% tax rate, isn't that the FIRST win?
Whether you later actually take withdrawals from the Roth or not, there is no ongoing tax there either way, the SECOND win.
If you DON'T do the conversion, and instead every year pay the tax due to RMD and SS, the excess dollars from RMD that you don't need for expenses goes into a taxable account, which generates taxable income every year... also at a higher tax rate because of the tax burden caused by the RMDs and SS. So the "unused" RMD left-overs are being taxed too, and at the higher rate.
Everything you mentioned has been taken into account, and the result is negative in our lifetime. It only goes breakeven or positive when the kids take withdrawals.
We have other sources of taxable income (two DB pensions and rentals), so we can't do conversions, inside the 15% bracket, in amounts necessary to completely eliminate RMDs or the 25% bracket. All we can do is reduce RMDs and the spillover into taxable. And that benefit is very clearly insufficient to offset the upfront tax and associated growth impact.
Doing higher conversions into the 25% bracket doesn't seem to make any sense at all, unless I think the kids will be 28% or higher.