AFAIK the 757 frame is too heavy to be powered by the LEAP engines. Those planes were powered by a class of engine between the old 737 and 777 engines, and nobody makes them anymore because they're not in demand, so a 757 MAX is just not financially viable.

The article said Boeing is talking to Rolls Royce for the new plane. American Airlines used to have a fleet of 757s powered by Rolls Royce engines assembled in Montreal Canada. I used to work on those engines many many years ago.

You can use the most powerful LEAP engine on a lightened 757 “neo”, it’ll just be a complete dog like the A321 and not a rocket ship like the old one.

Just ‘cause I’m totally ignorant about this stuff: why is that?

The 757 was powered by rb211-535e4 which had about 40k pounds of thrust each.

https://www.rolls-royce.com/products-and-services/civil-aero...

The 737 MAX is powered by CFM LEAP which has about 30k pounds of thrust each.

https://www.cfmaeroengines.com/leap

The A321 is an elongated/bigger version of the A320. Similarly to the B737 - which also went through this process a few times (e.g. compare the B737-400 vs B737-600 vs B737-MAX) - it means the engines are moving a heavier&bigger plane.