Actually Borghetti cancelled the remaining orders for 777's when he turned up (if I remember correctly there were firm orders and options for up to 13 777's) and then introduced the 330 and the ATR72-500 and 600, bought Skywest for an outrageously inflated price thereby introducing 3 more types to the mix and then bought tiger, so I think the blame for fleet complexity can be fairly pointed directly at JB.
Virgin had two types when he turned up - the 737 and the ejet. He gave the 170's away only to replace them with ATR's which cost as much if not more to run.
Not only did he add fleet complexity of a colossal scale, he did so at premium prices because he had to have it "now".
It is easy to arm chair quarterback decisions like I am doing above, but blind freddy could see that many of the decisions made were expensive ones and ones that will stay expensive for quite a while to come.