"What were they thinking - then" is not as important as "what they are thinking - now". One fact that clearly is on the radar screen of everyone in Congress, regardless of role or affiliation, is the tax expenditure for pension contribution and earnings is now around 1.4% of GDP. It is slightly less than the expenditure for employer provided health care. Combined, they represent 1/3 of the total of all tax expenditures and almost 3% of GDP, and 3/4 of our total federal spending deficit.
Tax expenditure means the value of uncollected taxes from the tax break. So, earned income shielded from taxes using IRA + 401(k) + similar programs, along with the investment income.
The message in this particular tax law is pretty clear. The people that earn the income and are able to shield it from income tax when it is earned should expect to pay taxes on it eventually. Should those funds outlast them, their beneficiaries will pay the remaining tax owed, and they have 10 years to do so. There simply is too much untaxed income in that bucket.