26.5 ....
if you see a barberpole and it decreases in value as you climb
what you will see is one of
(a) IAS constant during the climb from ground level. This reflects a simplification of the real limit. During the climb to crossover height, the Mmo limit starts somewhat higher (so it is "hidden" by the Vmo limit) and progressively reduces. At crossover, the limits reverse and Mmo becomes less than Vmo .. so the barberpole starts reducing at constant Mach.
(b) IAS slowly increasing during the initial climb to crossover. This reflects a constant EAS .. the rest as for (a)
If its Vmo, and caused by pressure forces on the aircraft, shouldn't it go DOWN with altitude since there is less air impinging on the a/c structure?
.. the problem with flying at too high an EAS is that the aerodynamic loads imposed on the aircraft's surfaces (and underlying structure) by the airflow can become too high and things can break. A bit like driving in your car with an arm out the window .. the loads on your hand/arm increase significantly as your roadspeed (EAS/IAS for an aircraft) increases .. then again if you fly Tigers you will understand exactly what I am talking about here .. One of the concerns is the pressures exerted on skins as the air runs past ... eventually the skin either gets pushed into the structure voids or, more likely, gets pulled off the attaching rivets ..
Your concern about altitude is tied up with a constant EAS resulting in an increasing TAS ... it does get a tad confusing until you are able to put all the different speeds in their right places ...
Dream Land
The barberpole is a combined Mmo/Vmo gauge so whichever limit is limiting is reflected in whatever the needle is doing.