I agree that currently no solution seems to be imminent, but I also suspect that is due to the bitter row between Great Anglia and Stobart.
However, have you tried driving from SEN to Leigh on Sea station? It can take 15 mins, it can take 40 … The A127 and the A13 both have to be navigated and it's not always pretty and prone to delays, especially around busy times.
I still think if the local council, airport and MPs all got involved (which they are doing for this C2C initiative) but focused their energy on the existing station then a solution could be found.
As an airline or a passenger I would more than raise an eyebrow at being told my passengers (or myself) couldn't use the station that I could see but had to get a bus …