You are not joking.

> Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within 60–90 seconds.

For the curious: https://en.wikipedia.org/wiki/Armstrong_limit

That's what I was thinking, climbing up there burns some fuel, and only so much can be recovered into speed through a slow descent which airliners typically don't do, they hop between Flight Level.

So I suspect only long flights will be operated that high, and there will be some calculation for the optimal altitude for shorter flights.

Stalls are very bad when the aircraft is close to the ground. Stalls at about 5000 feet or more above ground level can be recovered from reliably by professional pilots, which is why (combined with the existence of autopilots and the fact that it is the cheapest way to cover distance) the majority of the thousands of airliners in the sky right now are right up against the coffin corner.