No seems to be mentioning the elephant in the room.
A large chunk of the 1hr travel catchment area of CWL is in an EU Convergence area - it's GDP is less than 75% of the EU average (most of these areas are in Eastern Europe).
Having a successful airport and a massive tourist industry won't fix this problem either, because if it did Malaga in Spain wouldn't be in a convergence area (AGP has around 12.5million passengers a year).
Unless the fundamental issues are addressed CWL will struggle, except those problems are very difficult to solve. So the easy route is taken of state ownership of the airport and then trying to get APD powers to effectively state subsidise the airport at the expense of a privately owned and successful airport (IMHO the very reason why APD wasn't devolved).