Buzz, you are totally correct.
However, the system as it stands is far from safe.
Example 1 -
VFR flights are not required to cruise at a Quadrantal level. VFR flights are also not required to set 1013 when above the Transition Altitude.
Consequently, there is a high probability that an average VFR pilot will at some point each year forget to reset 1013 and adjust level before crossing at the base of an airway defined as a Flight Level.
The fact that this is seldom reported I believe is because;
a) Much of the airways workload at the distances from TMAs where the base of airways is in the flight levels, is at higher levels and there is few if any low and slow possible conflicts that bring the controllers attention to the bust when that attention is busy elsewhere.
b) VFR pilots ( by luck, and UK weather) for the most part cruise at the lower levels (3000ft and below).
Example 2 -
A VFR pilot sets the regional QNH and in keeping to the rules, flies at 4450ft below the base of an airway where the base is 4500ft.
All very nice until we check the regional QNH and find that it is 4mb lower than the QNH being used by the traffic on the airway!!!
30ft/mb erodes that 50ft....or even 100ft and places the flight within the airway despite being operated in accordance with the rules.
Regards,
DFC