I think you're missing the point, here. The 757 was a 727 replacement because the 737 was never meant to be flying the routes and carrying the pax that it is today. The 737 MAX 10 has become, pax wise, a direct competitor of the 757-200. Airlines are demanding a 757 that doesn't require a new type rating, which is how you get this frankensteins monster of an airplane.