Boeing could have turned the 757 into something that would of sold for years shame they dropped it
No they couldn't. They put the -300 out there for three years and sold how many?
In the bigger picture of things, the market the B757 was chasing was a niche; thin transatlantic. Transcontinental US missions could be done year round by the B738 and the A321.
Any further improvements would have required wing work and a new 40,000lbs engine, a sector which no engine manufacturer is currently active in.
Most B757 European charter missions can be done by the A321 and once the A321neo comes along, virtually all of them will be covered.
There's simply no market for a B757 in the modern world; Boeing saw right to cut the line.