Bear in mind that the Goblin used to be served by 8 car trains, and as can be seen on the platforms all along the line, they used to cater for these. In the early 1960s the through summertime trains to Southend, stopping at all stations along the line, were formed of 2 x Class 127 4-car dmus. This length was no different to what had been used in steam days.
It's quite apparent as you go along the line, indeed elements of the old platform structures are still there, all weed strewn, fenced off and crumbling. The only exceptions are the likes of Gospel Oak, where new platform designs have been introduced since then.
I don't understand why lengthening the 378s from 3, to 4, to 5, is a saga. That seems quite reasonable as traffic builds. If the contract with the train builder has been written so the extra vehicles are overpriced, that's just a poor commercial practice. And if the trains are so packed out, for so much of the time, then the fare revenue from all those passengers must justify providing sufficient accommodation for them. The bulk of the costs of train operation, such as crew, are fairly fixed, so it becomes more economical per passenger if you have to run longer, full trains.