If say Holding company A owns 100% of the shares in Airline A, which is based in the US. They could only sell the Airline to a company that has US based airline(s).
If HC A no longer owns any airlines, they should be able to start up new operations where they want. But if they want to sell or buy again, they are restricted to the market where the Airline is based.
This means that players can start up in smaller regions, sell of the airline, move to a bigger market with their cash, build a new airline and sell it off and move over to even bigger markets.
I think the idea of being able to buy and sell airlines is a brilliant idea and would fit well with the longer running Airwaysim gameworlds.
It would even help the exodus of players mid-game. More competition and it adds a third dimension.