There are still lots of legacy BB shops, and those shops are still hiring new employees, replacing lost or outdated phones and so forth. BB may be selling 6 million units per quarter, but if they are losing marketshare rapidly (as they appear to be), those sales most likely aren't worth much. Additionally, if they are staffed up for 20 million units a month (eg: tons of office space, lots of personnel), then 6 million is not enough to sustain the company. We've already seen signs of this in the form of massive layoffs.
Also, this isn't a market where you can stagnate, so they won't be around indefinitely unless they have the R&D team to keep pace with new trends, plus create a couple of innovations.