It could be argued that by not purchasing more new aircraft now, Joyce is doing future management a very big favour by not lumbering them with ongoing depreciation costs for aircraft that may become obsolete/inefficient in less that the required 20 year life of the aircraft. He is allowing future management the flexibility to purchase the most efficient and suitable aircraft at that time.
The problem with that argument is that how long is to long? Depending on what happens with the 737, if you wipe out a portion of your fleet with some AD then should have you waited 20 years to find a replacement?
By delaying purchasing new aircraft management also benefit from some supercharged bonuses in the meantime with their resignation occurring just in time for fleet replacement.