no way to liquidate an airline (i.e., buy it and close it altogether) The closest thing right now is to merge it in to your airline...however all of the subsidiaries routes and gates are merged in with no mass way to delete the routes or terminate gate contracts. It would be nice to be able to select routes from the checkboxes on the routes page and have a "discontinue/delete route" button. A mass way to delete gate contracts (discontinue service) at airports would be nice, too. Perhaps a way to lease out unused gates, too?
if you buy an airline as a subsidiary, and they have outleased planes, there's no way to reclaim them and sell them off. They also seem to get assigned to routes that you make money off of (the only way to cancel those routes is to delete the routes, if you can determine which ones they are). They will age with no way to fix them, and you'll get lots of notifications about the conditions of the planes.
how about an option to sell your subsidiary to another airline?
normal stock trading should not be available for subsidiaries (how can you own them and own no stock?). Two options here... 1) when you take over an airline, their stock should be listed as 100% owned by your airline (less realistic) and not tradeable or 2) you should only have control of the airline while you own more than 50% of the stock -- if you sell below 50%, you lose control of the subsidiary. When a company spins a company off, they usually retain some ownership percentage.
when you purchase 100% of a company's stock, you should should not have to pay any more money to acquire them. When you hit 100%, you should just be asked if you want to merge them, liquidate them (and keep the proceeds) or have them as a subsidiary.