You have a very valid argument. But the history of tax policy is fraught with examples of things that were "never going to happen."
When the 16th Amendment was ratified, income only above $3,000/year ($4000/yr for couples) was taxed at a rate of 1%. At $50,000 year, it went to %2. Only about 3% of the population was subject to an income tax. How did that all work out?
https://en.wikipedia.org/wiki/Revenue_Act_of_1913
Until 1984, Social Security benefits weren't taxable. That sure changed.
The only constant thing about taxes, at ALL levels of government, is that it will change. Now I'm not saying Roth withdrawals WILL be taxed in the future. But it sure can happen, if history is any teacher. Never is a very long time.