Burt Rutan talks rather passionately about the lack of innovation in space flight, but also mentions how fighter jet (maximum) speed performance has stalled:
In fact, from what little I know regarding fifth generation fighters, not only has the maximum speed stagnated, but it is degrading.
Traits such as stealth prohibit variable intake geometry and variable wing geometry, the maximum speed appears to never be used in reality, it is probably costly and creates other problems (we only have to look at the SR-71 here). Furthermore, other engine characteristics are probably more desirable, such as supercruise and good acceleration in the range where this is critical and actually useful.
However, in this question I'm asking what, approximately (mach 4, 5, 6?) would the maximum speed likely be, if maximum speed was allocated a similar budget and priority akin to that during the cold war era?
The reason I feel this question can be answered at all, is that I imagine there are some very real, practical problems with very high performance aircraft that require extraordinary solutions to overcome. An answer that identifies where these approximately are would thus go a long way towards a satisfactory answer.
Basically, the crux of this question is to identify if there have been any significant aerodynamic, engine, or other performance related advancements that would enable a fighter jet to travel significantly faster than e.g. the F-15 or MiG-25, while still retaining the same functionality as namely the same types of aircraft.
Similarly for a bomber, it is well-known that sustained mach 3 flight can be achieved by looking at e.g. the XB-70 or SR-71. Even with designs from the 1950s. What is realistic today?
Just to be clear: I'm not expecting an exact figure here, but if there are any studies that have looked into this, it would be a very interesting read. If there are no studies, perhaps someone would want to take the challenge and offer a somewhat comprehensive answer as a conjecture anyway. Any math/physics would be great.
I assume that in order to even begin to answer this question, it must be defined what a "fighter jet" is, must it have an air breathing engine, or can it also have a rocket engine, akin to the X-15? Since I'm asking the question, I will also take the liberty to arbitrarily define this here: The engine type is irrelevant, there may be more than one engine, of different types, of the same type. Whatever would make sense in the context of a fighter jet, or even a bomber. Furthermore, it must be a manned aircraft. If it can reach suborbital flight and beyond, like the X-15, it is still an aircraft (again, arbitrarily defined).
An answer that separates the air breathing and rocket engines as two different categories is fine.
This definition of making if fighter jet/bomber is mostly based on wanting to exclude unmanned experimental aircraft, even if hypersonic aircraft such as the X-43 probably serve as a good reference, I feel they have little root in reality, having no life support, no weapon systems, and perhaps no practical purpose whatsoever beyond data recording.
It may also seem like a pointless question, but this might be quite useful in terms of bridging the gap between aircraft and spacecraft, or any other application one might think of -- suborbital passenger jets, etc.