And many people will still go to a private tax service to verify that everything that could be accounted for is accounted for. The concern isn't that people wouldn't be able to make changes, the concern is that people would naively accept whatever they are told they should get back, even if that isn't the case.

To be fair though, I think this is somewhat less of an issue after President Trump's tax reforms; with the doubling of the standard deduction, it's become less fruitful for most people to attempt to itemize their deductions.