hmmm... where to start??
it must be just as easy to make a 50-seat long range aircraft consume as much fuel per person as a 300-400 pax aircraft.
Not really. The bigger your aircraft is, the less fuel per pax is used. The A380 has the lowest seat cost/mile of anything commercially flying.
Obviously, the 2 pilots up front's monthly salary had to be carried by fewer passengers, but that could probably be offset by the smaller planes having pilots with less seniority and therefore less pay.
Perhaps, but even so, a Captain, FO and a couple of FAs aint cheap for 9 hours at a time.
why are there no ERJ 145 long haul planes taking care of the smaller routes like London - Nashville, for instance?
Firstly, slot restrictions. If you wanna fly your aircraft out of LHR, which is going to make you the most money? - 300+ paying pax, or 49? You could feasibly go to other airports but that would mean more infrastructure and staff, as well as a smaller catchment area. Think about it, if you want to travel with BA to the US, no matter where in the UK you're from, you fly from London. If they started sending aircraft out from the local airstrip, its going to reduce the number of pax traveling on the 744s out of LHR, hurting profits even more.
The hub system has developed because its cheaper, and ultimately more convenient. With a full service airline you can connect from anywhere to anywhere, making the network a vast amount larger than it actually is. This allows more pax traveling in fewer larger aircraft, reducing cost, reducing ticket prices, generating more pax etc.
Why do you think hiring a private jet is so expensive? Partly because they cost a bloody fortune to run!!
There are a million reasons why your plan wouldn't work. Those are just a few!
And think about it, how many people do you reckon from the UK wanted to go to Nashville today? I'd be willing to guess not a huge number.