There are around 1.5 million people living in Essex and (my guesstimate) another 2.5 million in the north-east quadrant of Greater London. All these people should be able to access SEN within an hour or so. But there’s a problem. They can also reach STN or LGW in that time. Or even LTN / LHR depending on what side of the catchment area they reside.
The real problem is SEN needs to be able to attract people; to offer them a choice over other airports. Having this really nice airport is great but with 3 or 4 departures a day only a handful of people will find it works for them. It’s going to take something really special to start growing again and finding the momentum that took years to build up.