I still do, and I still mantain an open source Twitter client replacing the official one (no longer supported, as most apps on the platform). Q10, best phone I've ever had
I can safely say that you can't make money out of this platform anymore, the user base is just too small. If I had to live with the donations coming from my apps for BB10 well, let's say I would have already starved to death ;)
I'm one of those guys, I still use a Q10 (OS10 family) as my daily driver. I felt in love with its ux e.g. the hub which acts both as notification center and timeline, full gesture-based navigation (back in 2013!) and an app permission system which allowed you choose which one should be granted to the app (again, back in 2013! Android had to wait years before this was implemented)
Apps are scarce, many don't work anymore due to obsolescence, but the main ones are there: I mantain a Twitter and a Twitch app, and keep updating them to follow Api changes. Spotify and Whatsapp can be used through the android layer, the native BB maps are still functional. Another nice guy on Crackberry mantains a youtube app.. I feel I can say that if you don't have many requirements it's still a solid phone, it can't do much but what it does it does well.