It's a good decision to have some tasty rumours about.
For a start what are the airlines hoped to be getting rid of the 757s going to replace them with ? I would guess most of the anticipated source will be the US trunk carriers. But the 787 is really too big a leap in size, while the 737 replacement is too far away in time. The A321 has been a favourite in Europe but not so much in North America. As there are many years left in the 757s currently around, and the bulk are not that old, I wouldn't expect too many of the larger US fleets would be sold off any time soon.
I believe many at BA regret selling off most of their 757s to DHL and wish they had kept more of them. Some some were old, but others not.
FedEx will presumably want to go for just one 757 engine type, wonder which one. The RR is the all-round favourite but that impacts on the secondhand price of them.
Why this rather than more A310s ? Well there aren't that many of them left, and they're scattered thinly across many fleets (and few on the US register) which will lead to all sorts of compatibility problems building up a large fleet. Plus it's a widebody which probably means FedEx would have to replace all their ground handling equipment whereas the 757 is the same as the 727 for fuselage cross-section etc.