In practice, most people do not actually understand how the boxes on W-4 translate into withholding amounts. There'd be a lot of trial and error involved if done manually.

Regardless, there's a reason why I marked that issue as "not a big deal" -- because it isn't -- the other issues I mention are the meat of it.

The issue you appear to have marked as not a big deal is calculating the gross-up that would be required to keep employees whole not the per-paycheck withholding cash flow issue, which you labeled a “real burden” for some (unless I’m misreading your post). I agree both are manageable and would be automated if things switched models.