The big 3 may order the 757-300 in time; remember, AA and UA did not put the 752 into service until 1989, long after the type went into service with EA and BA. I think that if the operating costs of the 753 are as good as Boeing says, some if not all of AA, UA, and DL may look at the type for domestic operations. They will be watching CO and NW's experiences closely. Also, at some point, domestic carriers will begin to need larger aircraft for short and medium haul routes in and out of their hubs, they cannot simply keep adding frequency as airports are getting more and more crowded, so, in time, the 753 may gain popularity.
Concerning boarding and deplaning, Lufthansa did a study with 753 on its short haul intra-germany routes. They chartered a Condor 753 and flew it on short haul routes normally flown by the A300/A310. They reported that the single aisle layout of the 753 did not affect ground times in any significant way, Interesting.
LH is considering the 753 as a replacement for the A300/A310 for some of its short haul german domestic and european operations. No order yet.