While those who believe DTV may have been doomed ever since the local Govt sold the airport to Peel, it's worth considering a separate aspect. An airport is simply a transport facility - it does not have a magic right to exist. A passenger airport is sustainable only if there are enough people with enough money who want to fly places. Running and maintaining an airport is expensive - if those costs cannot be funded ultimately from there being lots of passengers paying money to the airport via their tickets, then the airport will slowly shrink and die.
Where are the growth businesses with corporate cash to pay for business fares ? Where are the international corporate headquarters ? Where are the companies selling software to rival silicon valley ? Where are the locals earning over £50k per year with a high propensity to travel ?
I seem to recall reading that Redcar was one of the most vulnerable areas of the UK following the credit crunch in 2008. The NE of England doesn't have anywhere near as many people or as much economic activity as Manchester - to equate the two is just a dream.
Even 10 years ago, there was barely a real need for DTV to exist in addition to NCL. The UK mainland has far more passenger airports than Germany despite its much smaller population and land area; there are simply too many passenger airports in the UK, and economics has simply asserted itself over time.