There are many carriers which have made a significant business out of mid-life fleets. Take Jet2 in the UK, which built up a substantial fleet of 15-year old 737-300, 737-800 and 757. Only recently have they moved on to their first new fleet purchase of 737-800 from the manufacturer, but at the same time are still expanding their mid-life fleet further. The new and mid-life aircraft appear to be used turn-and-turn-about on comparable duties, and the dispatch reliability appears similar as well. It would be interesting to see the lease rates and maintenance costs.