I am getting in the same situation with over W 30 million on IAD. However this is planned to prepare for future expansion of the game which is going to cost a lot in gold planes.
My strategy to save up on gold planes is to fly my own planes to just under the C maintenance limit and sell. And replace it with a new airplane. My current fleet at IAD is 12 BAE 146, 24 B737,54 B777 and 4 A380. To replace all of them in the future I need 12 x W 44.000,- (W 528.000,-), 24 x W 96.000,- (W 2.300.000,-), 54 x W 375.000,- ( W 20.025.000,-) and 4 x W 445.000,- (W 1.780.000,-). I am currently waiting for the player-owned Cargo planes which will use the rest of the money.
And NGO is still in a buildup phase. But that has to finance it’s own operations.
So the W 30 mill is just sufficient 
.
And keep in mind that the ultimate goal of the developers is to provide a game where you can have your own airport! I sure want to have some bucks when this opportunity presents itself.
Holding on to what you have may be the best strategy for now.