Considering that this is not just any ordinary, run-of-the-mill software update, but rather a merging of all platforms into one unified code base, it isn't surprising that the development cycle will take longer. Getting the code to work across all kinds of computers, 2-in-1 touchpads, phones, and other devices is not a simple thing.
So the short and not intentionally sarcastic answer is: Windows 10 Mobile will be released to the general public when Microsoft is confident that:
1. The installation process will work on as many devices as possible with a negligible chance to brick any of them.
2. The software itself will work on as many devices as possible with as few bugs as possible.
3. Any carriers that are still standing in the way of the OTA push of the software release have been appeased or circumvented.
Right now, this looks like the 10586.107 release is a good candidate, since the native W10M phones are now getting Redstone builds. This is a guess, but I think items 1 and 2 that I listed are probably completed, and it's just item 3 that is holding things up. Once the carriers have signed off (and they should since at least the 950's are running on AT&T's network already) then we should likely see something from Microsoft regarding a release date.
All that in mind, I'd rather they don't announce a date that they can't meet. I think there would be more complaints if they actually announced and missed their specific target date than if they just released a general time frame and had to push it back (as they already have) for whatever reason. Until then, I've been very pleased running the Insider preview release, so for all practical purposes it has already been released to my phone. This just means I'm not having to wait for an official release to start using W10M.
And firing people from the development team would likely cause even further delays in getting the software released, which in turn would probably cause the customers to start calling for your job as CEO. Just something to think about...
This thread will likely get closed and get redirected to:
W10M release dates answers inside