Since then Vmax has been declining, as the aerodynamics and mechanical complications ( e.g variable intake ramps ) of higher-Mach flight were determined to be less useful than transonic manouevrability and sustained supercruising.
The exception to this trend has been the superfighter category ( F-111, F-14, F-15, F-22, Su-27 ) which have maintained the same ~ M2.5 Vmax due to their specific role. Yes, even the F-111 was meant to be a fleet fighter.
But none have pushed up past the heady M3.0 level that was routinely broken by a series of prototypes in the 1960s.
With die sizes as small as they are, we have a problem where electrons...jump...through basically solid walls from an electrified wire to an unpowered wire. Now, turning on one circuit means the circuit browns-out and a neighboring circuit gets half-powered.
That is also what is happening with computers - it is simply more efficient to cram out more instructions per clock cycle than it is to cram out more clock cycles.