Around me (lots of expats with kids) the iPad is a killer to give your family back home instant connectivity in as much maintenance free and foolproof package as you can get. ( no cable, no malware, no driver, understood and supported by your internet provider, always on, minimal learning curve )
My point of view is the opposite, how is it useful at all for tech people. I never found any business case where either laptop, desktop or smartphone was not more convenient (the exception is reading, but I'm part of that minority that prefer to read on their smartphone)
I remember getting a tablet a while ago with a Pixel Qi screen that essentially doubled as an e-reader... wonder where that technology (and the Qualcomm Mirasol etc) all went.
iPad 2 is still a good device for reading and browsing the internet - hence people just don't throw it in the trash after 2 years and buy a new one like phones.
I don't really see that. I own a Galaxy Note (read: HUGE phone) and yet I still own a tablet. A larger iPhone won't be as large as the Galaxy Note, it just isn't Apple's way, at most it will be similar to the Galaxy S5.
To me the iPad has a niche that it covers (e.g. reading books, larger "table" games, etc) and even a large iPhone won't cut into that business much. Even if the iPhone was Galaxy Note sized I'd still buy an iPad, only maybe one single use case would shift and that is a minor one.