and the costs of maintaining and operating those old, thirsty and lets face it unreliable 300,s must be very high, and as they age even more the costs only going one way
Very low cost of leasing or depreciated ownership means they're not that expensive, though they lack reliability with age. They are not much older than BA's B737 fleet and the same age as some of BA's B744s.
Jet2 seems to have a similar philosphy.