We are a dev shop with a lot of time dedicated into BBOS and I'm wondering if RIM is planning on sending it to the graveyard after BB10 is released or will it continue to exist in lower end phones?
Nope, just like OS5 and OS6 they will have teams dedicated to keeping the platform around. It won't likely receive regular updates anymore and the OS will be phased out over the next two years I would think, but there will be millions of owners out there for years to come.
