You’re also making the implicit and incorrect assumption that this assumption of future income is not already the status quo. It is. The IRS already does this with your automatic withholding. It just does it at a higher level than necessary, due to what I mentioned above. Withholding basically assumes that a person will continue to earn whatever their paycheck was in every future payment period (weekly, semimonthly, whatever).

Your assumptions on how this would be dealt with are not realistic. The outlines of how to implement this not only already exist, they are already used and have been used for decades. All the IRS needs to do is glue together its knowledge of your income sources and lower the withholding amount based on being able to predict an individual’s income far more accurately.

Data on seasonal income is known too, for the record. Consistent trends in income changes with parts of the year are not a surprise to the government agencies that care about it.