I'm kinda looking forward to an ultra-long world too, but I can also see the many potential problems people have raised. The current worlds seem to stagnate after a few years anyway - would a long world be more or less likely to do this? I'm undecided. And it's a very long experiment if it doesn't work out.
As a more realistic idea - how about varying the dates a little. I know they differ a bit each time but there are certain fleet types that just aren't worth bothering with because they come too late. For example, in JA nobody seems to bother much with A300s and people are still flying 200+ fleets of Caravelles when there are 100's of 737-200Adv sat idle. This time around I am rich enough that I wanted to play with a few Concordes but they arrive on scene just one year before the end of the game so I can't be bothered. It'd be nice if the next JA ran into the early 80's.
How about a compromise - a 30 year world?
One issue with the ultra long game worlds that I am not sure has been addressed is fleet renewal. If we start in 1950 and continue for 70+ years, players will be faced with the big props of the pre jet era, first gen jets (Caravelles, Comets, DC-8's, 707's), second gen jets (DC-9, 737-1/2), First gen widebodies (DC-10. L-1011. 747, A300 classic), third gen narrow bodies (A320, 737-3/4/5, MD-80/757) second gen widebodies (767, 744, A330/340, A300-600), fourth gen narrow bodies (E-Jets, 737NG) and third gen widebodies (787/A350/A380) which is a LOT of fleet renewal even if certain generations are skipped. With the huge penalty for carrying a fourth fleet type, this will drive costs sky high, especially if the "legacy costs" that were discussed in other threads are included.
A possible solution is implementing a "fleet retirement" classification with regards to commonality costs where as long as a fleet type is constantly decreasing in numbers (maybe no less than 1% of fleet removed per game month) the fleet type does not cause a penalty. For example, I have a fleet of 737-200's that I am replacing with 737-300's in the 1980's. My 150 strong fleet of 732's is being replaced at a rate of 1 every 25 days as new 733's arrive, so instead of a penalty of up to 300% (as was noted in MT5 and JA5), all the airline will pay is the normal costs.