Everybody's situation is different, but my first Z4 was a new '04 which I paid about 37k for by passing up on options, and getting under dealer invoice. I replaced it at lease end with a new '07, which using the same approach, paid about 40K for. When the '07 lease expired I really wanted a 2010, and worked the dealer hard trying to get the cost down to what I thought was reasonable (for me) for an occasional driver. The lowest I could get a car for, and that was only if I ordered it, was about 52K. Adding CA taxes it was too much for my conscience. I agonized over the decision for months - I ended up getting a new 370Z Roadster Touring stick at $500 under invoice for 39.5K.
At that time a lot of posters were moaning over the price increase. In my view BMW had blown it, because they had pushed the price up to the Boxster level (depending on how you equip it) while every single comparison I have ever read between the Boxster and the Z4 picked the Porsche, but the BMW no longer had the $10K price advantage. When BMW just about stopped shipping sticks, I think they realized they had lost the performance buyer to Porsche.
Back to the thread topic, if you want to know which of your 4 points has caused the drop off in sales, compare the Z4 with the Boxster. that will answer your question. I haven't seen Porsche sales numbers, and I don't know if Porsche publishes sales by model.