I think a few here are getting confused between 'coffin corner' and airliner max altitude, and as 'airfoil' rightly says, the real CC is a 'dead' end. There was no controllable way out of a true CC. An airliner, however, with a few knots between high speed and low speed control problems is a different kettle of fish, and of course we can all fly our way out of that. If the U2 had a 10kt margin as BA said, that also was not 'CC', but just a place to be, how shall I put it, a leetle bit careful. It is not really clear what the OP meant when the thread opened.
I suggest that those 'younger' posters do a bit of historical research on the early jets and their problems, particularly the early straightwing mil jets. A lot of useful lessons were learned.