This doesn't work in AS... In real life, you don't have airlines just quit and go into bankruptcy. in AS, players can do that, so if there are three players in the market with evenly market share, and two players decide to leave AS one after another, so the only player left will control the majority of the market. in your theory, you want that player to go down as well because he/she takes more than 50% of the market?
It might be beneficial to take sure one airline don't take the slots in one airport, but controlling market share is simply not feasible in AS.
Edit:
Even if using your method, you need to have multiple players in the same market at any time, which is not quite possible. AS can't force anyone to enter the market unless they want to. Again, you have tons of airlines in US market in real life, and each of them is running the airline differently. After the mergers in some many years, you really don't have airline run the same way as others; similar airlines already had the merge. so even if it is legal to have a merger between those large airlines, i still really doubt they will do that. The management of the airline will be a mess right after the merge.
TWAAir,
Market share is a symptom of the problem not the problem itself. The problem is your current ability to run thousands of flights out of every major airport. This is not realistic as the American regulators would never allow one company to control that much of the marketplace.
9 &10 Bar US Airports:
ATL, BOS, BWI, CLT, DEN, DFW, DTW, EWR, FLL, IAH, JFK, LAS, LAX, LGA, MCO, MIA, MSP, ORD, PHL, PHX, SEA, SFO
That is a count of 22 airports. And if we were to include the 7-8 sized hubs that you could make a decent hub out of the number is around double the above. Now you tell me how it is justified that one airline holding company hold over 50% of the slots in most of those airports above. It simply does not make sense. No administration in the US would allow a company to become essentially a monopoly. No way!
Now, from a game play perspective the most effective way to limit someone from growing into controlling ALL of the above airports is through a cost function. Every time someone wants to go over 500 flights in an airport they should have to designate it as a hub and pay a fee. For the first hub the fee would be completely nominal. But by the time you add your 10th hub it would be enormous ($100m + 10m / week in costs say) This cost would NOT STOP you from becoming a mega-airline it would just slow you down. (Instead of adding $100 million worth of planes after a large company fails you're suddenly facing $100 million in costs to first add the hub.
Now, is that an artificial way of essentially adding in a regulator? Well...yes. But unless AS wants to have to constantly monitor players it is a solution to the Monopoly Problem.
I don't even suggest that this is done on the existing servers. Simply doing something like this on future servers would keep those servers interesting for a much longer time. (IE...more players last longer & late game entry is easier)
Oh...and call these costs, "Political Lobbying & Political Action Committee Costs" going to the Feds and local governments and of course the media.