This happens to every airlines as their fleet ages. I haven't really retired planes by myself yet but I think retiring planes at age of around 30 must be recommendable. Then buy new planes if you have financially stable enough. You won't face having to any planes when you just started operations.

Just try to 'become' one of them by adding lower cost flights. If there are already low cost airlines operating there just make prices lower so that any new competitor wont be able to add routes without bankrupting themselves. Even other spam lines eventually face intense competition and they sometimes miss highly demanded routes. Being a spam liner only annoys other players and isn't always bad as long as you don't use exploits. I'm not that experienced but I already experienced my airline growing very fast financially by adding routes even with competitions. You will always have to look for highly demanded routes where competitions are inevitable. So face it or risk going bankrupt.

I try to survive in world other than sandbox worlds, but I keep going bankrupt even though I try playing just like I did in the sandbox world. Less airports are vacant and I keep losing money even though my aircraft reach 0 hours available so that I cannot get a new aircraft. Does anyone know how to go financially stable outside of sandbox worlds?

It could be the aircraft or route itself. Check for any political restrictions, runway lengths, or too far of a distance which may affect it

I got the runway length correct by clicking add routes on aircrafts tab and other planes available in my fleet still appeared. My a330 had full 140 and was just delivered.

I'm trying to add routes for my a330 and it won't show up. The system won't even tell me why.

so when a world resets all airlines in there are gone?