You've asked a Short Question with a Long Answer, I'm afraid.

Peter, why do I suspect that when asked the time, you tell the inquirer how
to build a watch?

Evaluating airplane performance, especially from the stuff published
in the Popular Press, is a tricky business.

Big snip.

In the mid '60s, wanting a higher performance Interceptor for Southern
Florida, the USAF re-engined some of the F-104As with the J79-19
engine used on late model Phantoms. This had a non-afterburning
Static Thrust of 11,900#, and an Afterburning Static Thrust of
17,900#. With that much power, the 750 KEAS airspeed limit was
reached at all altitudes, from Sea Level on up, and the 250 Degree F
limit was reached from 20,000' to the maximum ceiling of around
66,000'. The ceiling continuously increased from 51,00' at Mach o.9
to 66,000' at Mach 2.0. It could very easily have flown higher and
faster, if the airframe limits were ignored.

Nice try, but untrue. The 750 airframe limit was not a factor above about
40,000 feet ... it was not reached at "all altitudes." (BTW, airframe limit
IS a factor ... was? ... for the SR-71 at intermediate altitudes.) Inlet
temperature could be an issue at the extreme top end ... Skyburner F-4 had
and Greenameyer's F-104 was to have inlet water injection ... but we're
talking 2.5 plus here.

As to "very easily flown higher and faster" the J-79 would experience burner
blow out between 65-70,000 feet and the engines would have to be shut down
approaching 75,000 because their minimum fuel flow settings would be too
high and cause overtemp. (Greenameyer intended to modify the fuel control
and use specially formulated fuel to allow the engine to run longer until
shutdown required in his zoom climb.)

To simplify your response, most older designs had high mach as a primary
design goal and thrust/drag created large PsubS "bubbles" past the transonic
drag rise region (F-104 a prime example, original F-14B ... glove vanes and
inlet scheduling intact ... another). That excess power in the 1.4-1.6
region (usually, SR-71 was much higher) allowed higher service ceilings
while supersonic.

Current design emphasis is on subsonic performance with high Q (indicated
airspeed) but not usually high mach as a bonus of their high thrust/weight
ratios. No large PsubS gains once above transonic drag rise. Ergo no
improvement in service ceiling supersonic.

R / John