As a Series 6 & 63 agent for many years, I would argue against my party's idea of tampering with the 401K structure simply to bring in more revenue as they try to "pay for" their new tax plan.
I support contributions to a 401K retirement plan with pre-tax dollars rather than after tax dollars (Roth) for most folks. Delaying the taxes until retirement allows compounding on more money for all those working years, and then paying the taxes when most people will be in a lower tax bracket. The RMD (Required Minimum Distribution) finally requires withdrawals from this nest egg when you reach the age of 70 and 1/2 in most cases. This amount withdrawn is added to your gross income for the year as taxable income as you work thru your tax liability. The remaining balance of your nest egg however keeps compounding and growing tax deferred like it always has for all those earning years.
So in other words, the continuing growth on your account can help pay for the taxes on the part that you now have to withdraw each year, including the part that you have delayed paying taxes on all those years. Now that is truly the Magic of Compounding Interest beyond its technical definition.