I wonder how much of a deterrent the A127 is?
It's effectively the only access road to SEN (unless you count the A13) and it is always a trial.
The airport is 18.5 miles from the motorway network, no idea if this puts people off but I suspect most passengers would book their flight first then worry about how to get to the airport afterwards. I have used the A127 and for much of its length it feels like a motorway anyway.
The railway station is the airports biggest plus point but the continuing lack of trains in the morning and at night must have some sort of impact if someone books a flight and then has an issue with a lack of public transport if they haven't got access to a car or doesn't wish to drive from the big smoke.
The subject has been flogged to death here but it is the airports Achilles Heel as the railway link is its greatest strength and its greatest weakness.