Is making faster military fighters even practical?
In the 1960, faster fighters were seen as desirable as they could outrun 1960s technology missiles to an extent (see the reports of SR-71s being fired on but never hit by SAMs). Of course, missile manufacturers saw this and developed faster missiles and longer-range radars to negate this advantage. And as it is much easier to make a missile faster than it is to make a large aircraft faster, that's an arms race the airplane manufacturers always lose. 

While high speed allows you to outrun missiles, it also reduces maneuverability: at Mach 3, a constant-G turn will be much larger than turning at the same G level at Mach 1.