When you try to predict the energy it requires to propel a missile 80nm then you have to plan for a 3-dimensional flight path that evolves thrust from the motor and actual lift from the missile body, two design limits for the missiles size and shape. Too much lift and the missile requires more thrust. Too little thrust and the missile falls well short of the mark. More thrust requires either more volatile fuel or more quantity of fuel, both paths requiring substantial boosts in the missile weight in size.
The design could be launched at a higher speed to boost its energy without requiring more fuel or energy, but that puts new constraints on other parts of the plane for tracking and whatnot. One interesting note about AMRAAM is its reported 50% boost in range when used by the F-22 in supercruise flight due to its higher launch velocity. As a side note, the F-22 will likely roam at a combat ceiling far in excess of current fighters which will only add to its survivability.
Larger missiles are less manueverable and require either a larger sensor to track its target or a preprogrammed flight path within the vicinity of the target. (The AMRAAM uses inertial guidance with periodic flight corrections from a datalink.) You are pretty well confined to active radar homing seekers for the missile as infrared and post-optical sensors would become far too stressed in even only 30nm of flight to be of any usefulness at the effective range of a 80nm missile. The more command guidance from the launch aircraft the more vulnerable the missiles accuracy if the launch aircraft is interupted - such as by a counter threat. (One of the reasons the Sparrow missile had a 10% chance of kills in its early life was precisely due to the pilots unable to maintain missile lock in a high threat environment.) Modern USAF and USN systems do not require the launch aircraft to remain locked on the missile, rather the missile is commanded by a vast system wide datalink. You don't want to rely too much on external command because what happens when you don't have GCI or AWAC information in the combat zone?? Plain and simple you lose almost any chance of survival!
The reason the SR-71 was never officially shot down has more to do with politics than with actual ability of the Russians or Chinese to down them. I don't know where you get the idea that a plane can survive Mach 6+ flight in anything but a one-way trip to hell, its hard to say. The truth is the SR-71 was too vulnerable to SAM technology by the mid-1980's to operate them over several Third World countries, let alone the Soviet Union or China! (Even more laughable is your assertion that they merely push the throttle up... ROFL! Too funny.) The CIA and USAF lost a fair share of them to probable unfreindly fire, although some were too mishaps. You'd never expect to have the USAF admit that their pride and joy was vulnerable to the evil empire, would you!?! Then again the CIA would never admit they even operated them...
I've seen the B-2 in real flight. As soon as it takes off you have less than a minute of visibility. The pilot makes a series of quarter-turns every five to six seconds, on his way to his cruising altitude, and with each jink it becomes increasingly difficult to maintain eye contact. And this is well under 10,000 feet in broad daylight! The truth is that the B-2 is one nimble and powerful machine for being only "subsonic"!