Max speed
Quote "Why have heli main rotor blades not seen this kind of change"
Lost again
Look at some recent published changes in rotor upgrades and you will find improvements of up to 15% (load/efficiency/speed)
By compairison : Some winglet addons speak of 3-5% improvements (cruise efficiency, MTOW etc).
So in my opinion it is not all that bad, my R44 many times cruises higher and faster than a PA-28 that I am overtaking.
It is fair to state that due to the complexity of rotor-wing design, optimisation is quite difficult because all the different constraints that have to be met, even fair that we may be close to max keeping the same architectural choices. As you will see on different threads on this forum major break through will probably come from different designs.
Skytorque
True but the published enveloppe is mostly simplified-conservative.
My R44 sim predicts quite well retreating blade stall, and there are many regions outside the published envelop that will not create problems, altough I am not advocating here to ignore the published envelop for the reasons you quote.
Conversely the R44 rotor blade problems in the Andes (high/heavy/fast) probalably were within the published enveloppe, but the systematic hitting of one of the boundaries can create fatique problems.
A very practical limit (irrespective of envelops) : vibrations are a very good health sensor I think, even if one theoretically would be within the enveloppe.
d3