Kent's population is growing massively. There is a lot of migration from London out to the east and construction of a few large new towns in the pipeline.
For the people in Kent Gatwick or city airport are the best options at the moment. Gatwick involves the M25 and anything can happen. Southend or Stansted means crossing the bridge so forget it.
For the freight side of things post Brexit there my be a demand for more international freight coming in direct so I don't see why this can't work. The airport is right next to the M2 and the rail link via Canterbury and Ashford onto HS1 as per the existing Ramsgate train is viable. Especially if done at night when the passenger services aren't running. There is also a ready made harbour right next door which could easily feed the incoming international frieght into the EU.
The only spanner I see is the new Lower Thames Crossing which will bring Southend into play for Kent.
The project is viable so long as investment in the transport links is forthcoming. Post Brexit I would hope that would be the case with these kinds of projects. Even as an HS2 supporter I would gladly see the project put on hold in order to prioritise the construction of international trade infrastructure short term. Unfortunately at the moment the UK seems obsessed with causing chaos building 'smart' motorways that nobody wants.
One thing Manston has going for it is big support from the locals. That is rare these days in terms of airports and while there are a few Nimby's it is generally regarded as an asset to the county. Post Brexit we are likely to see an abundance of land go up for sale and Manstons value as an airport will possibly be more than as housing.