Blackberry Classic User over here
I run two phones, a Sony Z3 compact and a Blackberry Classic (that's the newest one with a keyboard running v10). What's surprising is that I tend to use the Blackberry most due to the keyboard, I can type without looking, I can hang-up a call without taking the phone out of my pocket. I have a few android apps on it which mostly work ok. For taking pictures, GPS navigation and reading web sites it's the Sony every time - this isn't the majority of what I use my phone for though.
After I think two years on v10 (I had a Q5 before) though I still don't find the gestures intuitive.
I think Blackberry make quality hardware and, if they went to a proper native Android build I think they'd get a lot of business customers who actually type emails on the move - you know the original reason they were popular. No one else makes a decent Android phone with buttons anymore.
They could be sensible and release a developer OS release for an existing device and see how many people try it. They can maintain that it's unsupported as is the case with the unpublicised Android app support currently.