United may well be looking at the A321LR as a replacement for the 757, however I'd expect Boeing will also be looking into the feasibility of a 739LR. They wont just leave this market to Airbus.
I don't think future withdrawl of the 757 will have too much of an impact on EDI. I can't see ORD lasting, and most other 757 routes can be done by the 737MAX. The 767 fleet will be saved for routes into Europe beyond the range of the 737MAX, so I doubt that will be making an appearance at EDI any time soon.