Worrying indeed ...
It is a given that one will need to come down to more "consolidated" air. Assymetries at high altitude ... and it is very easy to lose control of the ship.
But not being able to maintain altitude on a twin, with the other engine at MCT, being it at 15000', or 10000', or 5000', is indeed very worrying.
It seems that Certification needs a very good review of requisites or specs, and this is for both sides of the Atlantic.
The very first accident of a 767 was a deployment of a reverser inflight at 20 something thousand feet. Flip it upside down, went into a dive, into overspeed, and inflight break-up.
It seems that Boeing had tested the aircraft with a reverser deployment inflight, but it was at a much lower altitude, and it was controlable. At high altitude it is impossible to control.
Got tired of doing 8 or 10 hour sectors, so decided to move to something smaller, the european turboprop of the Airbus group. Max certified altitude is 25000' and it can take 68 pax.
Sometime ago did a flight with 7 pax onboard. It was a bit longer sector, so we decided to go to FL240. From FL220 upward it climbed at 300'/min. Thought to myself ... WTF, with 7 pax onboard it only does this ? Spoke with an examiner that did some testing on the aircraft. He mentioned that on an empty aircraft, with an one engine at idle, the aircraft barely climbs.