If future tax rates do change, though, the Roth IRA will result in more wealth when tax rates rise in the future, while the traditional IRA will benefit when tax rates are lower in the future. Though in practice, because the tax burden for a Roth IRA is paid upfront – when the (after-tax) contribution is made – there is no future uncertainty with respect to its future tax rate; instead, changes in tax rates primarily impact the future value of a traditional IRA, in particular, making it better or worse off depending on whether or how tax rates change.
To cope with this uncertainty, one popular approach is to ‘tax-diversify’ between the two types of accounts, splitting contributions between traditional and Roth IRAs so that there is at least ‘some’ benefit regardless of which direction tax rates go (as higher tax rates benefit the portion of dollars in the Roth, and lower tax rates would benefit the traditional IRA dollars instead).
However, the reality is that splitting dollars between traditional and Roth retirement accounts isn’t just a form of diversification; because the outcomes are correlated to each other, the net result is that tax diversification doesn’t actually diversify the risk, it simply neutralizes the opportunity altogether. Or viewed another way, splitting between traditional and Roth IRAs is more akin to just bailing out of stocks altogether and owning zero-return cash.
If you want to see a more thorough discussion of this topic, here is an excellent article.