It's a matter of convenience. If there was a "deluxe" ps4 that would play ps1, 2 and 3 games, I'd spring the extra cash for it. Some of us have wives who don't want several boxes cluttering up their living rooms and frankly, it can be a pain in the ass unplugging everything then plugging in a new machine.
Edit: also, from my perspective, an all in one system would be extremely handy. I'm living in Japan, but before the end of the ps4's life cycle, I hope to move to Canada permanently. Imagine how much more convenient it would be if I could just copy all my saves onto a hard drive, sell the consoles then just pop the hdd into a new ps4 in Canada. I got rid of the boxes for most of my old games and cart them around in numerous cd storage boxes. I know that most playstation owners aren't going to uproot themselves and move across the globe, but it would be very convenient for me should the option be available.