Entitlement benefits -- the vast majority of mandatory spending - are not 'already incurred'.If you "revise" your decision to pay obligations that you have already incurred
All entitlement spending can be cut; the only mandatory spending that cannot be cut is debt service.
At the moment we are only taking in about half the revenue we need to pay all our bills. Thus, obviously, if we don't borrow the rest we are going to default on about half of our obligations. Basic arithmetic.