Honestly I wouldn't be surprised if the IRS could already have a 90+% success rate for 80+% of people. It seems like the real issues would be in the very rich who can take advantage of many loopholes or the average person when big life events happen (things like death, marriage, and birth, though most of this could be automatically reported as well but easier to fall through the gaps).
Maybe I misunderstand, but it doesn't seem like the IRS needs any additional information than I have presumed it already has. This information is at least all known by the fed, so it doesn't seem like a data leakage. And I'm someone that highly cares about privacy.
I suspect that the real pushback for return free filing is from 1) tax filers like Turbo Tax who would lose a lot of business and 2) the ultra wealthy as RTF would put pressure to simplify the tax code and reduce the number of loopholes.