I have to say the numbers on CFR where always very low and so I am not surprised it has gone.
I do think there is potential for MAN, I know a lot of drivers of demand in the North West for travel between the NW and the essex area, and the journey is like torture by road or rail and totally unreliable. As I have said before, everyone immediately jumps on the "serving London" bandwagon, but there is a lot more to the route than business folk going for a meeting in London (although if you did want to meet in the east of London or even the city, then it may work out quicker than the train)
Point to bear in mind - Not everyone lives in, or wants to visit the centre of Manchester or London - lots of people in the suburbs and surrounding areas of both airports will find a flight much more convenient that travelling all the way into a city centre just to travel out of it again ...
As for the gatwick comment, I think Gatwick - Manchester did very well on numbers, that wasn't the the issue, the issue was BA's lack of capacity and the ever increasing value of slots at Gatwick that caused the route to end, added with the very high operating costs of Gatwick ... it's why FlyBe pulled out and ditched highly profitable routes such as LGW-BHD, IOM etc
Southend is a totally different ball game ...