Physical Keyboards fit into a niche market currently occupied by Blackberries, and yes that niche happens to be in the enterprise world.
There are times I would love to have a physical keyboard attached to my 8.1 phone, however there are also times when I don't want the extra bulk of it.
I think an option to utilize Bluetooth keyboards would allow a niche market to have "covers" for the back of the Windows Phones that have a "slide-out" or "slide-out and tilt" physical keyboard that can be operated via Bluetooth. This would be the ideal situation, as it would make the nonkeyboard phone of choice available for average consumers, and allow any of the models to have an "attached" keyboard for that niche market. I am hoping that W10 for phone will take us in that direction. Perhaps not a huge increase in market share, but between the security of Windows Phone, the integration with Windows Laptops and Desktops running Windows 10, and adding in an attachable Bluetooth keyboard, it could spell doom for the Blackberry market altogether.