There will never be a "FINAL" release of a software program for any software company unless it completely stops doing any updates. So while FSX went "FINAL" a few years back, Companies like Laminar and Lockheed are moving forward and hopefully will continue to. None say that you have to keep up with the Betas or even move onto the next version. the point here is progress. 10.20 is the 64-Bit version and when it comes to being a "Stable" version, this when the 3PD's will commit to building, although they already have for the most part and are awaiting the final tweaks.
I hope that there will be a 10.30 and 11 and 12 and........ as it means that the program is either getting better or staying up with technology as it changes which will then change your expectations. If you want a "Stable" release then my suggestion is for you to untick the Beta Box in the updater.
So, while there currently is and has been a "Stable" version for a while (10.11) and soon to be a 64-Bit (10.20) version that only x-plane has, I hope there will NEVER be a FINAL version.