Assuming that the documents that are tabled with interim supply are as you described them, there is no reason why we, as parliamentarians, couldn't require Treasury Board to prepare interim supply estimates based on projected future. They would have to do those calculations anyway, and then there would be no loss of information. You're assuming that information is lost here, but I don't think there is a requirement for any information that currently exists not to be presented to parliamentarians.
On June 20th, 2017. See this statement in context.