Even at twice the speed a bomber or recon aircraft would not be safe over 1980s Russia let alone 21st C Russia.
Most models of S-300 are mobile so a fast flying SR-72 could be cruising along and suddenly find itself flying between two radar sets for S300 but that a S-300 battery has been set up between those active units with its radar off waiting for it to fly over so it can shoot you down.
The speed of sound is generally accepted as 320m/s at sea level... so mach 6 would therefore be 3 x 320m/s or 1.92km/s... old S-300P could only engage targets flying at 1.2km/s, but the S-300PMU1 and PMU2 can engage targets flying more than twice that speed at 2.8km/s.
S-300V could engage targets at such speeds too.
S-400 and S-500 can both easily engage targets flying much much faster than just under 2km per second.
No point in building a "Mig-41", interceptors are (currently) a thing of the past. Besides, the Russian aerospace industry is busy with the PAK-FA, newer variations of MiG's and Flankers, and the Fulcrum replacement.
The Russian Air Force wants PAK FA and may eventually look at a light 5h gen fighter to make up the numbers without being too expensive, but it is not really interested in a replacement for the MiG-31.
The Aerospace Defence Forces however find the MiG-31 to be a very useful aircraft and really don't need an expensive stealthy plane.. they just need a double MiG-31... ie twice as fast... more missiles, longer range, bigger radar. they don't need stealth or manouver capability.
A light modern fighter is far from decided. They might just go for a UCAV.
Very true, though I think a light cheaper 5th gen fighter would sell rather better than a UCAV and would be more acceptable to the conservative Air Forces around the world.
In the meantime they can spend their time trying to master ......mach 1 with F35
Or indeed new technology like oxygen generators so they don't suffocate their F-22 pilots...