Think of the 757 as a 707 with two big engines versus four little ones.
The 757 was originally intended as a direct replacement for the 727, but Boeing grew the concept to meet the demands of airlines that wanted more seats, more range and better takeoff performance.
The MD80, 737 and A320 became the true 727 replacements and the 757 was developed into a unique niche.
After some years of life, a few airlines figured out that the 757 would be fine to connect thinner city pairs across the Atlantic and since they already had the aircraft, then why not?
The day of the 757 flying across the pond had begun and will end only as available frames run out of hours and cycles.
The soft product on this UA flight will be as good as what they offer on any long flight. The hard product will be okay, although one difference between now and the 707 days is that you'll have much less seat pitch.
You won't necessarily have more room in the back of a widebody either.