How to deal with slow deliveries

Right now I'm Delta Airlines, in world S4B, and what's happening is that my 757-300 fleet is starting to expire lease-wise. That basically stops me from now expanding further, as I'm renewing at the same rate they get delivered. Should I extend the leases and wait for the 787, or order A310/A330s to fill in the gaps?

Maybe buy off the leases?

ok, but when I do that, how do I expand further

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It's the aircraft age factor I'm thinking of. Cost isn't an issue currently

Looking at your airline, the average age of the 757-300 fleet is 1.41 years, and for your airline, it's 1.14 years. I wouldn't consider age a problem at all where you are right now. That being said, your reputation will drop as age increases — ie. running twenty-year-old aircraft is not a good idea. I personally tend to replace my aircraft when they reach around 10-15 years of age (depending on their versatility) — that way it keeps the average fleet age (the important metric) down. There are exceptions, such as the Saab fleet (which I keep around for longer because they're useful and cost-efficient) or niche aircraft like the 757-300, which I know is a low-cost, high margin plane that ceases production. 

...or you could ignore reputation altogether (you're in a sandbox world, so you won't even get any awards) and just not replace your fleet at all. besides, sandbox worlds only run for 20 years, and i've run 25-year-old aircraft without any problems. focus on getting new planes and establishing new routes instead.