It's hard to predict how the tax preparation industry would be disrupted if the IRS started preparing returns based on information already given to the government about your personal income and deductions. 

Many countries listed in the article that do pre-populated returns do not allow itemized deductions, only a personal exemption. When you think about it, counties that collect property tax from home owners do not provide the IRS with tax payment information. 

I think taxpayers would become uncomfortable if organizations and government agencies started submitting payment information connected with itemized deductions to the IRS. Under the current tax system, even if the IRS pre-populated returns for taxpayers, a large number of home owners or individuals in states with high income tax rates or people who make large donations to nonprofit organizations would all disagree with the return and file a rebuttal return.

On the flip side, the IRS would likely NOT assume that a taxpayer qualifies for Earned Income Credits or Child Tax Credits, etc. and these taxpayers would also send in a rebuttal return.

Without a major overhaul of the itemized deduction scheme for higher income earners and the popular tax credit scheme for lower income earners, it's not unreasonable to assume that the same 80% of the population would employ an accountant or use a tax software to file their personal income taxes.

https://thinkprogress.org/amp/p/e2df8036ac25?gi=1e6b0b2abe3c