The main problem and also a constant headache for airliners still operating the 757 its the lack of proper uld's // containers for cargo hold, quite the opposite of even smaller airbuses, (pax config) ... Presently I'm not 100 sure about it, I must confess, but some years ago all 757 holds were bulk free luggage, no uld devices, except for those freighter built/modified on purpose for cargo carriers.
So no need to do any reengine neo à lá Airbus for a plane still using philosophies of last century...