We lived in France for 10 years up to 2011 and have friends near Lyon. We visited them earlier this month and arranged the dates so that we could fly from SEN. Previously it would be from LTN or possibly LGW depending on price at the time. OK, I'll always use SEN in preference to any other LON airport even if it cost a little more but I think our experience of using SEN this time would apply to any traveller. SEN is not our nearest airport and many of our friends use STN, LGW and SEN with equal readiness. LTN seems more rarely used for some reason.
We stayed overnight at the Holiday Inn and have friends in Southend so met them there for a meal. Next morning it took 20 minutes from leaving the hotel room to sitting down near the departure gate (plenty of seating) including bag drop. The flights were on time on the ERJ-195 outbound and the RJ85 returning, both equally comfortable compared to an A319/320 (perhaps more so on the ERJ). On arrival back at SEN we were on the M25 40 minutes after touchdown including baggage reclaim. Overall a very pleasant comfortable experience.
My first point is that any traveller on those flights who had thought he would give SEN a try, perhaps tempted by introductory fares, would likely be equally impressed and ready to use it again and probably tell others of its merits. That's why market penetration is more important than yield initially when you're entering a mature, well catered for market. The revenue benefits come along later in the piece.
Secondly, Stobart will have made money from our stay at the Holiday Inn, the week's car parking, hold baggage revenue which they load themselves, in addition to their share of the fare and ancillary revenues. For any given basic fare yield these things add substantially to net profit or reduce any initial net losses.