denabol asked;
just how high can today's jets sustain cruise in commercial conditions.
The higher you fly, the thinner the air gets and the closer the stall speed and the maximum mach number get to one another until the margin between the two becomes dangerously narrow.
Some aircraft will run out of lift and/or power or have other operational limitations before they get close to this point. If limits weren't set, some aircraft could be flown to a level where they would be literally flying a tightrope between Mmo and stall speed. An extreme example of this was the U2 spy plane. If the autopilot or an engine fails in such circumstances, the consequences can be quite challenging to cope with.
In 1985 a China Airlines 747SP suffered an engine failure whilst flying at FL410. The autopilot did its best to compensate for the reduced and assymetric thrust but the control inputs increased drag and, with reduced thrust available, the airspeed declined. The aircraft began to fly at an unusual attitude as the autopilot tried to maintain altitude and counteract the effects of the engine failure. After a couple of minutes, the pilot disconnected the autopilot, but he couldn't react fast enough to replace the large control inputs that had been being provided by the autopilot. The aircraft departed controlled flight and plunged 10,000ft in twenty seconds and a further 20,000ft in the next two minutes or so. By the time it recovered at about 10,000ft it had pulled up to 5G and suffered considerable damage. The aircraft landed safely at SFO and no-one was killed. The accident report blamed the pilots for not handling the engine failure correctly so the accident wasn't primarily caused by flying too high but it illustrates what can happen when problems aren't handled properly whilst cruising at high altitudes.
Manufacturers and regulators set a maximum permissable altitude (service ceiling) for each aircraft type. This is intended to give a safe margin between Mmo and stall speed at the service ceiling.
The service ceiling for an A380 is 43,000ft (13,115 metres). 747s can normally go to 43,000ft; 45,000ft is the max. Some biz jets can cruise much higher than this e.g. the service ceiling for a Gulfstream V is 51,000ft.