This is true, they are based on assumptions regarding the future; however, the idea that ObamaCare will be repealed anytime before Jan 2017 is not so much an assumption as it is a guarantee. Only if the R's achieve a veto-proof majority in the Senate could it be undone prior to that. So this wipes out 3 fiscal years out of ten.
The better question is this...in 2009 the "stimulus" bill was passed which increased spending by almost a full $1 trillion dollars, approximately 25%. Why is government spending increasing at all? Who cares if it's "only" 3.4% per year? It should be 0% increase for the next 10 years. IMHO