How do you guarantee you are safely clear of any terrain, and unless using GPS how can you easily pin point your position to even know where you are?
Yes I have an IR, and on it, besides GPS I learnt how to use a VOR, DME and even ADF, didn't you?? ( in fact I even learnt Decca, but never used that apart from in the RN),add in radar assistance and it is not that hard to know where you are, Sir N has answered the other points.
As a matter of interest, how does an airway work if not CAS, it seems to me you are then just forcing all the aircraft into the same airspace, and if not controlled you are actually
increasing the collision risk!