Default starting aircraft changing? I think it is really a case of if it ain't broke, don't fix it. It starts to become too complicated for world creators to decide on and players to work out what's happening if plane types are changing all the time. But mostly, it isn't really a problem in the vast majority of games (5, 10, 15, even 20 year games).
It only becomes a big problem with the really long games. These are fairly rare, and usually fill up well before the first 10 years anyway. But that aside, even if you start very late, in AM virtually any aircraft can earn a decent amount of money for your airline, at least enough to buy a cheap 2nd hand one off someone else in the world and get your airline going with something new, even if this takes slightly longer than with a newer start aircraft.
Basically, very few games go long enough for it too be much problem, of them, most fill up very quickly, and even if you are in such a situation, you should still be able to make enough money to get your airline going.
Historical pax data...
It would be interesting to compare some of the data for GDP and airport passenger numbers, because I think they would likely have a decent correlation. If somewhere with a good set of historical data (like the US...) could be used, I think a decent model for airport pax size could probably be created just based on the variables of current pax levels, date and country GDP.
The assumption, which I don't think is too unreasonable, is that the entire country is at roughly the same level of development. It would also allow airport openings/closing to be simulated - by reducing their pax to zero when not open (in conjunction with an alteration to the LF script to make airports with 0 pax generate 0 revenue...).
Of course, this might need to be accompanied by a change in the LF/fare scripts to allow airlines to make greater income in the earlier days. As it stands, one can make a decent profit, but not a fortune, during early stages of the game, even with the huge capacity airports. Back then small numbers of people (i.e. small pax levels) were willing to pay huge fares to fly. If the data was put into the current scripts, you would be flying between airports with a few thousand pax - giving you quite low fares. I suppose this is related to the fact that pax numbers don't give a true indication of demand for travel between cities. In the past, there might have been a large amount of demand to go between two cities, but, as fares had to be so high (due to planes being more expensive to build/operate) this opportunity was only open to a small number of people.
In many ways the current method neatly simulates this. In the earlier stages of the game, as you have a 20 seat plane, when you fly between multi-million pax airports, you get fares into the thousands of Euros. This is actually fairly accurate - as in real life, people would have been willing to pay huge fares to fly. This is balanced up by the fact that aircraft are considerably more costly than comparative aircraft later in the game, the fact that you don't actually make much money per route (1000 x 20 people... less than a 100-200 seater will make you in modern games) and that they can only manage a few routes at most per aircraft (as they are slow). That said, there is still considerable room to make lots of money (as there is in modern worlds).
I guess the conclusion I would make is that you can make similar (on balance, perhaps slightly lower) amounts of money in old games as compared to modern ones. From a realism point of view, that isn't so realistic - airlines back then were far smaller than current ones. This probably isn't such a bad thing though, as I suspect the game would be quite dull if your fleet consisted of 20 aircraft that operated 2 routes each out of a handful of bases. Sure you can make monster airlines like in the modern games, but that is part of the thing that makes the game interesting to play.