My understanding is that there are several patents in this space that need to be licensed, coupled with everyone wants to copycat the best selling phones, and don't want to design a "niche" product that looks old fashioned.
Google's Andy Rubin has a patent on one design!
https://www.google.com/patents/US8488778
It's also difficult to design a slider with a large screen and not have the phone be unwieldy.
http://www.theverge.com/2013/11/15/5104704/i-come-not-to-pra...
"And that's where you run into trouble with physical keyboards, according to Motorola's Rick Osterloh. Keyboards "make it so you have to cut the screen size down, or have a slider form factor which adds a considerable amount of cost, thickness, and weight to the product."
That thought ought to scare current OEMs: I'm sure BB will "pivot" into IP Licensing thereafter. Then again they don't need to be sold before they start doing that
About the exiting hardware business, I don't think it will be too long before BB finally crashes down and gets sold. The differentiator for them has been hardware since they ditched BBOS for Android (not saying it was a bad decission, that thing was horrible), now what's left is some java apps and some branding to put on some random ODM... Until a Microsoft, Lenovo etc gets in and buys them for their IP