The Airport certainly won't work if everyone talks it down the whole time!
There are good reasons why people from the South East might want to use Manston. These are predominately to do with not having to use the M25 to get there, short check-in times, parking near the terminal etc.
However if the airport got too busy some of these advantages would disappear!
The real skill comes into determining how many of the 71% will in reality consider flying from Manston, and of course the first thing they need to know that is whether the flights are going to a destination they want to go too, and at a time they want to go.
The problem is that if you have a new or unknown airline operating routes out of Manston potential passengers will be wary about booking in case the airline goes under. Over time that fear would of course decrease. This means you either need a very well funded new/unknown carrier, or an established one to offer flights. The former will take several years to get established at Manston once services started, the latter would take a year or two.
But of course there are very limited sources of funding for this, particularly in the current climate.
Cettainly if Manston offered flights to a somewhere I wanted to go I would consider it, because even though Gatwick is a lot closer for me, using Gatwick is a pain. However I only fly shorthaul once every 2 years or so, so a lot of "me"s are needed!