I don't know that the 757 airframe, as well as most commercial aircraft is designed to go much faster than the cruising speed that airlines operate them. I know wing sweep has a lot to do with it. The max speed difference between a 757-200 and a 747-400 is less than 100 knots in the 747 favor.
Commonly "extant" is incorrectly used to simply to say that something exists, without any connotation of survival. "In existence" would be the correct way to end that sentence.