So let me get this straight. If SoS turns it down, that pretty much cabs the whole idea and we are left to fester while everywhere else grows. If the SoS grants it, the current owners are not even going to put shovels in the ground for another 3 years, which means we are left to fester while everywhere else around us grows. If the SoS grants it, but the current owners sell up, it will take ages for a new buyer to come forward, ages for them to take over, and then no doubt start doing their own cost analysis and assessments, which will take ages and we will be left to fester while everywhere else grows.
As WH states Secretary of State can rule it out due to impact on the green belt, but this is unlikely. Other than that, he can't "turn it down" - he can ask for it to be considered by a public enquiry. In which case, the application is heard again with arguments for and against.
The flaw in the rest of your argument is this idea that "everywhere else grows". Well if that were true, it would suggest a strong recovery from Covid and a thriving aviation sector in which case the business case for building the new terminal or expanding the existing terminal (assuming the planning application is ultimately rejected) will be much stronger than it is today