That's why if JetBlue had bought the 737-800 in the first place they wouldn't passenger load limits on transcon flights between the US West Coast and New York-JFK.
If Boeing had modernized the 727 by replacing the 3-man cockpit with the 757 2-man cockpit and changing it to a twin engine configuration, it's quite possible the 727 would have continued being the most popular jet airliner. It was only after the the 727 was discontinued that the 737 orders really started increasing.
The Next Generation 737 is almost a new aircraft compared to older 737 models. It has two totally new wings and new avionics. It has the same dimensions as the classic 737's so it fits at gates designed for older 737's and can use the same ground equipment, but in many ways it is mostly a new plane.
Actually, the A320's that are out now with the high bypass engines can do coast to coast without any problems. Our first batch of A320's (from Braniff II) had problems doing BWI-PHX on a hot summer day. But our newer 320's and 319's easily did BOS-SFO or LAX-JFK. We have a 319 that does Phoenix to San Jose Costa Rica without any problems, and we also go to Cancun from Phoenix without much problems (even less now that we have ETOPS).
I don't understand your post. I flew JetBlue last month from JFK to San Diego and back and there wasn't an empty seat on either flight.