Hi all,
Personally, I only enjoy playing the long worlds. They provide one with the time to develop an airline, fly ancient aircraft models and introduce new ones in their stead. It's a real journey through time!
Indeed, I prefer to remove inactive airlines. After a while, defunct airlines over-burden some already very frequented routes. Also once you reach the late '90s, you could end up operating an A-319 on a route which also has flights on DC 3's or Boeing 80A's or something, which is not very realistic.
I clearly state in the forums that bankrupt airlines will be removed, and most of the times (sometimes I forget) I send a welcome message to new airlines to let them know about this.
I tried playing shorter worlds, but for me its a frustration that just when I'm getting my airline started, the world comes to a sudden end in a nuclear apocalypse or something, and everything is over.