I don't think any of the A129 cam versions have been as trouble-free as the A139 has been up to the last couple FW's. One needs only to scan the threads on those cams to see the issues.
A119 V3, A129 Duo/Plus/Pro, and A139 have all been largely trouble free except for the odd fault in individual units and certain features,mostly added after launch like the buffered parking mode on the A129. The A139 has maybe had more issues at launch because it contained more features, but the basics have been better than previous cameras, certainly things like power supply, capacitors, clock battery, etc. seem significantly improved on previous cameras, very few reliability issues and plenty of lessons have been learned.
There does tend to be a lot of discussion on issues with these cameras, mainly because a lot of people buy them, and customers can often get features improved/fixed/added. Other brands that have issues often don't get discussed because there is no chance of getting them sorted.
Given the similarity of the A129 versions and Viofo's history, I would not be surprised to see whichever of those is the slowest selling model being dropped in the next year's time- maybe sooner. With The A139 2-channel emerging that gives more reason to drop one or two of the A129's. Viofo moves forward quickly and doesn't continue slow-selling models which makes sense business-wise. But whichever Viofo cam you choose they are all good cams so you can't go too far wrong with any of them provided you do some research to be sure the one you prefer isn't having issues with a function or feature you want to use.
Phil
A129 Duo is still a good camera for those happy with 1080 at a good price, not sure there is a reason to drop it unless they run out of stock and there are issues getting components, which may happen at some point. I recommend getting a 2K version, but that does come at extra cost and really isn't necessary in countries with bigger plates like UK.
A129 Plus is relatively new and seems to be doing well, so I guess that will keep going too.
A129 Pro still has the best video quality of just about any dashcam, despite a reasonably low price, so no reason to drop it unless they can produce something better for similar or less price... Maybe something to watch out for, but not just yet since it is still hard to beat that image sensor...
A119 V3 is the oldest I think, really should have wifi these days, but not having wifi does make it cheap considering the image quality, so maybe that will keep going too. For people wanting wifi there is the A129 single channel at not much extra cost.
A139 is the newest, so that will surely continue for ages, also a lot of people like the screenless design, so there is a market for both dual and triple channel versions.
As you say, they are all good cameras, and give plenty of choice, so they will all likely continue for a while... but in that case, what will Viofo come up with next? Maybe something to add to the collection rather than replace... It is a while since we had a real improvement in image quality, but Sony doesn't seem to have made any major improvements to their sensors, other than to make cheaper versions, so for a big improvement we still need larger sensors, which tend to be significantly more expensive.