It would be easy to discount all of this, but the tax savings could be significant. This is more relevant for people trying retire early. If early retirement is serious option for you both, I would consider allocating more to traditional 401K, especially if you're already maxing out ROTH IRA. You could then take the tax savings from the traditional 401K and invest it in taxable accounts. The whole idea behind this is to create buckets of accounts that you can eventually choose to withdraw from a strategic way if you want to go the early retirement route. This is would I'm personally working on.
Even if tax rates do increase and the brackets change, I think there will still be a good amount of income that you'll be able to withdraw from while paying zero or little tax on condition that you don't need to withdraw a significant amount of money each year during retirement (50K a person is tax free if you plan it right).
Today for an individual tax payers, you can essential withdraw 12K tax free because of the standard deduction that all Americans receive. Then you can receive 38K of capital gains tax free up to 38K. These amounts are nearly double for MFJ taxpayers. Therefore, if you only need 100K between you to or a little more, you would ideally want to have buckets of funds to choose from. If you have a high traditional 401K balance, you will have more flexibility as you can do a ROTH ladder conversion to convert some of your 401K balance to ROTH in a strategic way to have early access to retirement funds. You end up paying tax on a time where your income level is relatively low (that's when you would do this this conversion), but gives you access to the w/e you converted after a period of 5 years. You would keep doing this each year during the beginning of early retirement. After this you would want to withdraw from your a bucket of taxable investments (the non-retirement account) because a certain portion (around 78K for MFJ filers) would be tax free. After this, that's when you can go ahead and take from your existing ROTH contributions that you have already paid tax on a long while ago.
For me, I'm personally hedging against tax rates being a lot higher in the future so I want to diversify my options so in the future, I can choose where to withdraw from in case I need to early retire.
1) See the 4 points included here:
https://thefinancebuff.com/case-against-roth-401k.html (btw, I don't agree that the ROTH 401K, shouldn't be used - I just like the points that were brought up and people often don't understand the main point that in retirement you're filling up the LOWEST bucket first)
2) Then watch this very good video explaining the items I touched on above in a much clearer way, with illustrations:
https://streamable.com/5gqcs
3) Then read this for en explanation on the different scenarios between investing in only ROTH, only traditional, traditional with ROTH ladder conversion, taxable only investments, etc:
https://www.madfientist.com/how-to-access-retirement-funds-early/