The MIG-31 is a much more capable jet than the MIG-25, but speed remains the series' most prominent capability. Of course, the US/NATO are concerned of any improvement but we do not 'fear' the MIG-31 like how air forces observers often commented that we are.
Here you can compare specifications, production year, cost and pictures fighter aircraft.
armedforces.eu
To sum it up...
- The MIG-31 and the F-15 are roughly the same dimensions but the MIG is heavier empty and fueled.
- The F-15 have better range.
- The MIG-31 is faster.
- The MIG-31 have slightly higher service ceiling, but not enough to be tactically advantageous.
- Both jets have been used as test platforms for experimental weapons, such as anti-satellite (ASAT) missiles.
Most air forces cannot afford dedicated platforms, such as the B-52 or SR-71, so if an air force have to buy its defense, they would have to shop around for the most versatile platform as possible. Speed demons like the MIG-25 and the MIG-31 are not useless, but is your geopolitical situation necessitates getting over your borders as soon as possible? Think about that for a moment. If your country shares multiple borders, and maybe at least one of them is hostile to you, is getting to targets important? So it is not as simple as looking at hardware specs. Another issue is what if the seller does not want to sell, like how the US will not sell the F-22 but is selling the F-35 like ice cream. So how many countries flies the MIG-25/31?
In my opinion, I favor 'jack-of-all-trades' or generalists over specialists. Over time, I can raise the bars for those 'jack-of-all-trades', but specialists are sort of 'locked in' into what they can do. For the MIG-25/31, they are essentially speed demons.
Personally, I wish the PLA leadership is filled with idjits like you, but I respect the PLA leadership too much to know better.