DC3, as a Chelmsford resident I can say you're talking utter garbage and can categorically confirm that SEN is the airport of choice for Chelmsfordians. Perhaps Southend folk don't go to Chelmsford much but people from Chelmsford certainly go the other way to the seafront or theatre! But, as you note we're also only 30 mins away from STN. That means that while SEN would be the default for AMS, GVA and the Costas, if it's a choice between multiple daily flights to GLA for £25-40 or SEN's £60 minimum for eight flights a week of course STN will generally win. As STN would also win for the genuinely cost-conscious. But remember we're quite a wealthy city, and many can and do pay for the ease and convenience and this is where SEN wins. Or indeed LCY, which is also an easy journey.
DUB might well be cheaper from STN, but the SEN frequency is probably enough to get people interested. MAN might also just about be of interest to avoid driving or trekking across London.