Buster the Bear,
That was an interesting read, and somewhat logical. I think Flybe are aware of the same issue with their Embraer especially when compared to Dash fleet. And like Jetblue, they ordered the regional jets when the price of fuel was not the hot potato that it is today.
All this said, it to my mind seems logical that airlines may also need a smaller type, perhaps for route development, or for being able to compete with regional markets. E.g FR's 738 does not stack up on some routes and a smaller more cost effective type could be more appropriate, routes like LDY PIK and some DUB - UK routes when regional airlines like EIR are offering frequency and targetting a business customer. Though the key consideration for LOCOs will be keeping costs down overall by having just one type. I'm sure they have all done the maths on the scenarios...
EI-BUD