>The main thing we'd have to fill in ourselves would be cost basis adjustments for RSU vests.

The cost basis thing used to be a nightmare. At one point with some old stock that got converted to another company's stock with an acquisition, I just threw up my hands and put in a plausible number. (It wasn't a huge amount of money in the scheme of things.) But these days, the only time I've had to manually provide a cost basis to my accountant was when I was donating some stock.

I’ve never had RSUs show up properly without needing cash basis adjustment. Same goes for ESPP shares.

Yes, for a taxpayer who has a salary, a bank account and a brokerage account, and no other income than salary, interest, and investments, the IRS already has every number needed to compute the tax bill. With the last round of tax changes, very few people will have enough deductions (charitable contributions, local taxes, etc.) to make itemized deductions worthwhile. So the IRS could just do what many other countries do, which is to present the taxpayer with a filled-out form, which the taxpayer can either just accept, or amend (perhaps add deductions or unreported income).