The standard MiG-25s generally had IRSTs.... just like MiG-23s did, so having long range IR guided missiles made sense.
The issue however is that the R-40 family of missiles was designed for the MiG-25 so the designers expected it to be carried around on the 25 at very high flight speeds most of the time.
Many mach 2+ fighters will never fly at such speeds in their operational lives... it takes too long and burns up far too much fuel.
The MiG-25 and MiG-31 are interceptors so flight speed is critical so they often fly at high altitudes and top speeds as a normal operational day in service.
Of course their missiles move at between Mach 4.5 and Mach 5 so flying around all day at Mach 2.5 is not a big deal for them, unlike older IR guided missiles for which mach 2.3-3 was their top speed.
The purpose of carrying R-60s was because the MiG-25 is one of the fastest planes on the planet down low... it can move at about 1,500km/h which is actually rather fast at low altitudes so chasing down enemy launched cruise missiles it potentially an important role so any interceptors that return to base that need to be rearmed and also some of the aircraft initially sent out can be loaded with twice as many R-60s as any other missile they could possibly carry... the speed limit wont be so important as they will be engaging cruise missile targets much closer to their bases and flying at low altitude they can't reach mach 2.4 so it is not an issue any more.
The issue however is that the R-40 family of missiles was designed for the MiG-25 so the designers expected it to be carried around on the 25 at very high flight speeds most of the time.
Many mach 2+ fighters will never fly at such speeds in their operational lives... it takes too long and burns up far too much fuel.
The MiG-25 and MiG-31 are interceptors so flight speed is critical so they often fly at high altitudes and top speeds as a normal operational day in service.
Of course their missiles move at between Mach 4.5 and Mach 5 so flying around all day at Mach 2.5 is not a big deal for them, unlike older IR guided missiles for which mach 2.3-3 was their top speed.
The purpose of carrying R-60s was because the MiG-25 is one of the fastest planes on the planet down low... it can move at about 1,500km/h which is actually rather fast at low altitudes so chasing down enemy launched cruise missiles it potentially an important role so any interceptors that return to base that need to be rearmed and also some of the aircraft initially sent out can be loaded with twice as many R-60s as any other missile they could possibly carry... the speed limit wont be so important as they will be engaging cruise missile targets much closer to their bases and flying at low altitude they can't reach mach 2.4 so it is not an issue any more.