New engines and sharklets are logical for the a320 series as fuel is more expensive.
Many people argue that boeing is making a mistake with the MAX, as the design of the aircraft is outdated. It has been rewinged and re-engined so many times.
But they were forced to do this in order to come up with a fast solution that keeps them in competition with airbus.
The embrear ERJ was too small, not able to carry enough passengers.
What you are proposing is not realistic! There is no demand for a 757-like aircraft type at the moment. It can be very challenging to fill all those seats in todays economic environment. Airlines need high load factors.