humpback said:
Trust me, I'm not missing anything. You don't necessarily start with less in the Roth- you can contribute the same amount and make up the tax difference from other sources like savings, which is how many people (probably most) actually do it in practice. Running the numbers this way, you can come out ahead with the Roth even if your tax rate is lower in retirement.
But then in your example you are not starting from the same point. With a 30% tax rate now, as in the previous example, you have either 10K to invest pretax, or 7K to invest post tax.
To make a comparison by showing numbers based on 10K pretax and 10K post tax is not a valid comparison, as they are not equal amounts.
Now imagine you pulled the $3K difference from savings instead of out of your Roth.
You could have $10K to invest in the Roth that will grow tax free, or $10K to invest in the traditional growing tax deferred and $3K in a taxable account.
If I did that, then I am not making a fair comparison between the two options. That $3K difference was earned and taxed at some point. If we work with the 30% tax on that, it started as about $4,286 pretax.
Let's simplify to illustrate the point. You have $10K in pretax money that you can put away towards retirement. You have no other savings with which to draw on. You are taxed 30% currently. So, for these purposes, that $10K is all you have. You can either put the full $10K into a traditional retirement account, forgoing paying taxes now, but paying in the future, or, you can pay the taxes now and put $7K into a ROTH account. This creates the apples to apples comparison.