The problem I have with this argument is that there is a limit to the deferral. With a TIRA you will eventually run into the RMD date, which will occur no matter whether you need the money, and no matter what the current tax rates are, and no matter what the current state of the market is. IMO, and recognizing that this depends on whether you need the money in the IRA to live, the tax free growth plus the lack of a requirement to ever withdraw the money makes the Roth conversion the better choice, all other things being equal.
I believe there is a problem with the deferral argument even w/o the RMD limitation.........assuming tax rates at conversion and cash-in are the same.
Ex 1......same as given previously.....
A & B both start w/ 100K TIRA and 25K taxable side fund; both in 25% bracket
a)A converts to Roth, uses side fund to pay conversion tax and has 100K
Roth. N yrs later, the investment doubles and he has 200K Roth
b)B does not convert, has 100K TIRA and 25K taxable side fund. N yrs
later investments double and he has 200K TIRA and 50K taxable side fund.
Looks like he has more but he can't spend it w/o paying taxes within the yr.
He owes 50K taxes on the TIRA but actually has less than 50K in taxable because taxes are due on sale. Assume (charitably) that no taxes were due along the way for side fund and that it is a 20% LTCG tax on the 25K gain. 5K taxes are due so side fund only has 45K and B has total of 195K after paying 50K tax on TIRA.
What happens if instead of selling after investments double, you wait longer.
Ex 2: This time wait until investments are 25x larger
a) A starts again w/ 100K Roth and no side fund. A ends up w/
2500K Roth.
b) B starts again w/ 100K TIRA and 25K side fund. B ends up w/
2500K TIRA and 625K side fund. Making same assumptions as in 1)
side fund after taxes of 20% on 600K gain of 120K ends up w/ 505K.
2500K TIRA must pay 625K taxes so B ends up after taxes with
2380K which is even less than A has than in Ex 1 (both in absolute
and % dollars)........so deferring longer is even less favorable for TIRA.
Hopefully I didn't mess up the math too badly and, of course, the conclusion is 100% dependent on the specific assumptions of constant tax rate which may bear no resemblance to a person's real circumstances. TIRAs only look bigger because they are before tax and you can't utilize (spend) them
w/o paying tax.