New aircraft are like new cars. You go and buy yourself a brand new 5 Series BMW, and you're looking at 20%+ depreciation the moment you drive it out of the showroom. You gain what, exactly? Well, you'll have that new car smell, it will be under warranty (excluding consumables and wear-and-tear items), and you can show off to your mates down the pub.
It still does exactly the same as a 20 year old model of the same car. The new one looks sexier, and it has all those switches, dials and GPS systems that can pinpoint your mother-in-law's house and call down a Tomahawk airstrike. It's also built by robots and designed to fall apart in exactly five years - the marketing boys finally woke up to the fact that building durable cars was bad for sales. The electronics are designed to cease functioning exactly 24 hours after the expiry of the warranty, whereafter your car will spend more time in the workshop than out of it; in the process trebling the manufacturer's profits on the original sale.
The old one, on the other hand, will just keep chugging along. It doesn't look as sexy, and it doesn't have all the electronic gadgets that are the dogs whatsits; sure, it burns a bit of oil and you've replaced a few bits along the way - but it still outpaces most modern cars and carries as many passengers. It needs a bit of care and attention - and doesn't always start first time, especiall if it's wet or on winter mornings. So what if you can't afford to replace it as you'd only get £500 on trade-in ... it still does the job you bought it for when you paid £300 for it three years ago.
Most importantly, it drives like a real car.