Looking around on the internet it sounds like the cost of the aircraft accounts for about 10% of the cost of a ticket. Frankly, airplanes sound downright cheap compared to the cost of staffing and fuel. Doesn't seem like the right place to start looking for cost-cutting measures.
If you want to understand how important new aircraft are, you need to take more factors into account, such as turn times, average numbers of transfers, etc. into account. The airlines have been moving away from the hub-and-spoke model, towards point-to-point for quite some time; aircraft CPM plays a significant part in this shift (as well as in slowing it down).
