Seems the speed of Mach 2 has more to do with air resistance than desire. If the builders could get them up to Mach 6, they would. But the amount of fuel you would burn getting there would run you out of gas fast, plus, your airplane would probably melt.
Going more towards stealth (or stealthness) seems to be the trend. That, and as was mentioned before, pilotless.
Question I have is does all this lead to a whole class of air weapons that are little more than remotely operated missles? Very fast and designed to fly straight into the target while under operator control. Seems to be going that way in another 30 years.