> As of 2014, the tax code was only about 2,600 pages long

The usa hardly has a monopoly on tax code complexity. We're talking about personal income tax right now and I can't find a complexity index for that, but for business taxes the usa ranks fairly low on https://www.taxcomplexity.org/

> That’s not a nightmare scenario. That’s just a standard audit.

The "nightmare" scenario was "you recalculate it from scratch, send it to them, and then they audit you." - a scenario you put forward where this improvement to the tax code (auto-filled returns) results in a world where any time your return isn't what they auto-filled you get audited. That sounds bad, but has not happened anywhere where auto-filled returns have been implemented.