Why? The short answer is that they simply don't offer anywhere near enough value - the 2015 fits me and my work significantly better. Also, I despise the keyboard - the worst I've ever used on an MBP, or the trackpad, which is too big and can't reject wrist input (at least from me). The keyboard has been given a thumbs down by our ergonomics department, which means no one can order one for university use. There's more, but I've written it elsewhere, and don't want to repeat.
No I don't. My current laptop is a 2009 Macbook Pro 15" 2.66 Core2 Duo 4gb 500gb hard drive. I would imagine any macbook would be a big leap in terms of performance.
Yeah, I can imagine going from a 2009 model to a 2016 with a crazy fast SSD will have a nice jump in performance.
One thing I noticed in playing with the new models, is how quick it is. I clicked on the safai icon in the dock, and the app would open up before the icon could even begin to bounce. Other apps such as Word and Excel which are more bloated opened up blazingly fast as well.