Can high-bypass turbofans cruise at high altitude?
Yes in principle, but some modifications are advisable.

What limits the maximum operating altitude of a jet engine (besides the thrust needed to climb up there) is the length of the combustion chamber and the absolute pressure of the air entering it. Since the atmospheric pressure drops with altitude and the compression ratio of the compressor stays constant, the absolute pressure in the combustion chamber drops with altitude.

For combustion to take place, first the fuel droplets injected into the air stream need to evaporate. This is made easier by higher temperature and pressure, and the lower both are, the more residence time of the fuel-air mixture (caution, PPT link) in the combustor is required for good combustion. Longer combustors have higher pressure losses and weigh more, so engine designers try to limit their length.

Just throttling down can already cause the ignition to stop at high altitude. If a flame-out occurs, the engine cools down quickly and a restart at a lower altitude might not be possible. Turbofans suitable for high altitude operations use special measures to stabilize the flame. When they are in place, there is no reason the B-52 can't continue to fly at 50,000 ft.