Issue:
The problem here is that not everyone had the same experience. If five users had a lumia 1020, 3 of them could be having the best experience while running insider builds, but the other 2 could be having a nightmare with no end in sight (hard resets helped in some cases but not all).. Multiply this issue over the entire line of phones that were able to previously test w10m and report issues..
Insider builds:
For those phones running insider builds (so obviously you've chosen one of the rings), but haven't been picked to receive future updates, Treshold builds will be the last stop on the w10m train. Per MS there might be a few more updates, but most resources will be focused on Redstone. This means you can keep using insider builds, but in a few months support will end which effectively means that if there are bugs or whatever MS won't be there to fix them with an update
Which build:
For now stick to whichever ring you're on. I would go with production, because feedback for .164 builds could still matter and it's the most stable build for phones that won't get Redstone (well some phones)..
Back to 8.1:
Not a good idea.. Support for that will end as well. Think of this as the start of a new era (basically the wp7 to 8 issue). Apps that are on 8.1 might not be updated as frequently (or at all) by the devs. Most will be optimizing for w10 and just building for w10 as well