Great, sounds like TurboTax will still be able to compete if the IRS gives people tax filings precalculated, as TurboTax can say "We can save you $XXX!"
Except many people will try TurboTax or a competing solution anyway, just in case they can get a little bit of money back.
Tax evasion and overstating deductions are common partly because the IRS doesn't have access to the information necessary to verify everything -- that's the same information the IRS would need in order to pre-fill tax forms accurately for the 1/3 of taxpayers who itemize their deductions. 1/3 of American taxpayers a pretty big market for TurboTax and similar services to thrive in.