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).