One of the problems with the B757 is that it has a very long hold, meaning it is a long way to push a bag from door to bulkhead when it is only 44" high in the hold.
The B757 / B737 have three common loading systems for belly space - normal bulk, telescopic bins and sliding carpet. All of these still require every individual bag to be loaded onto the aircraft, which takes time and can create IR issues (becoming an issue in Europe more and more particularly).
All of these methods have prominent downfalls (as can containerisation with occasional internal roller system failure).
Normal Bulk Load:
Very labour intensive. Parts of Europe are looking at putting maximum kg uplift per bagsmasher / per day.
Take up a lot of room and are quite heavy.
If the system fails the hold is almost unloadable due to high-friction carpet (almost impossible to slide a bag - just as hard to carry them).
All of these systems have been related to back problems experienced in Loaders.
Although staying with a standard fuselage width (B707, B727, B737, B737NG, B757) has saved a lot on development and re-tooling costs for Boeing in the short-term, I am concerned about the long-term ramifications.
You can get away with smaller narrowbodies being bulk loaded like the A318/19 & B737-300/600, or if you are a low-cost international charter operator that can rely on cheap labour and long turnarounds for the larger variants (plus the B757).
However, if you were a CEO of an airline and told that you could:
A) load your aircraft with 7 containers using 3 ground staff, or
B) load 140 bags individually plus cargo with 5 people for a similar aircraft size.
which would you choose?
I am not proclaiming to be Pro "A" or "B" when it comes to manufacturers, infact, I would love to see Boeing explore a new fuselage dimension for its next generation of narrowbodies that can allow for containerisation. This comes from both being an enthusiast and an employee in the airline industry.