I think to avoid initial rush and for player numbers to match demand, make game world start with, say 100 player spaces (enough for 2 airlines in each top 50 airport - the only ones worth basing in these times). Every decade or so about 100 more spaces for airlines open up. Can either be done in batches of 100 on the turn of the decade (would have to be announced) or it goes up by 1, 2, 5 periodically so maximum player numbers will meet growth. F.E. Currently DOTM has 400 spaces in the 1980s and has increased to 600 in MT in the 2000s.
I second this. I think the game should start in 1950 with 100 players. Every month open up 1 more opening, so when the year 2000 roles around you'll have 700 openings. I think most players who have played the 1950's era already will end up waiting until the 70s/80s before joining anyways (I will).
The other issue will be airport slots. Growth is still explosive, which means something must be done to make sure those joining later can actually fly somewhere. The easiest way, in my opinion, would be to have airlines limited to 25% of the slots available at an airport for every time block. This means if there are 100 slots for the 500-600 hour, they are limited to only 25 slots for their airline in order to keep slots available for other airlines. The only other option to ensure late joiners have a snowball's chance is to have explosive slot growth to counteract the explosive airline growth. Airports could start with 50% slots and double every decade, for example.
Fuel is extremely cheap in the early decades, so if someone (like Curse) were to start in 1950, you could bet that when 2000 roles around he'll be king of the world. Fuel either needs to be very expensive or other costs need to be dramatically increased to counteract this.
One way to counteract the "legacy" airlines in the game is to add a new line to the balance sheet: retirement costs. Many airlines run in the red because of pension/retirement costs. After 15 years, airlines should start to incur pension costs that last another 20 years based on tenure. This means if you had 1000 employees in year 1, during year 16 you would have to pay pension payments to those 1000 employees (assumes they've retired). After year 36, those 1000 employees roll off the books, but the employees from year 21 (15 years prior) are added, which means you're likely dropping 1000, but adding 20,000 for a net increase of 19,000. A simple formula would be (number of pensioners)*(current average salary)*(80%). This would help counterbalance airlines from having wild profit margins and buying out their fleet, which ends up going straight to the bottom line. IRL airlines have to plan for these costs, why not implement them in AWS? This would also provide smaller startup airlines an opportunity to undercut the legacy airlines with lower prices. Add high fuel costs and you'll get the perfect storm for players to endure
The last part is the "boredom" factor. Players will get bored with a longer game world when all they can do is upgrade or buy out their fleet. I think the number of bases should be uncapped, but the staff costs increase exponentially, based on the number of aircraft based there. Strong players could challenge themselves opening a 10th base and a fuel spike could wipe them out. I think the challenge of growing an airline to the maximum size possible would be the holy grail for most experience players--imagine having the top 10 airports in the US as hubs with 1000+ planes and eventually collapsing completely due to rising retirement costs and fuel spikes.