While I agree with you in thinking that the new ATV will not have an A12 (at least not only a A12, I wouldn't go as far and say that it makes no sense for Apple to do it.
There could be multiple reasons for Apple to want to drop the A10X for the A12, although, I would bet that those reasons would have little to do with the end-user experience, and more to do with Apple's bottomline and/or long term SW support plans.
if it were long term support an A12Z would probably better match SoC configurations in terms of application and GPU core counts better than a A12. The issue with the Apple TV is that they are likely to go back into another 2-3 (perhaps 4 ) year lull where they do nothing. So need to pick a SoC that will also being up to the task 1.5-2 years from now with whatever the GPU load is in the future. A A12Z would probably do that better than a A12.
If selecting only from the 'plain' A-series then a A13 would have more longevity legs than a A12 in the Q1-2021 timeframe. Especially, in the context of long term Ai/ML code support.
Number of units sold doesn't do particularly much for Apple long term support. It is when the product was last for sale (replaced) . And the A12's in Phones are only being held up by the XR at this point. There is an iPad 2020 that will probably generate more sold.
Additionally, maximum "units sold" criteria couldn't really have been how the A10X got into the current Apple TV. Probably not going to be the criteria on the next one .
The A10X is not used any device on the current Apple line up, with the exception of the ATV4K, but the A12 is used on multiple devices currently on the line up. Apple could be dropping the A10X for the more widely used A12.
if Apple is out to keep the same price point on the AppleTV then soaking up the "paid for" A12Z production after the iPad Pro moves onto A14X would make sense. The A10X was only ever in one other product iPad Pro ( removing the largely minor variation on screen size).
Here are more rumors about a A12 ATV:
Multiple rumors have indicated that Apple is working on a refreshed version of the Apple TV, and a report today by Nikkei Asia Review adds to speculation that Apple will release a new version of its set-top box next year. Cites sources familiar with Apple's plans, Nikkei's Apple TV mention...
forums.macrumors.com
This seems to support the theory that if there is an A12 ATV coming, it will not be alone, and there will be more powerful models launched at the same time.
I think there is a strong desire among some that Apple move to a 'plain' iPhone SoC so that the price of the future Apple TV will drop. The 'rumors' only roughly point to something with a A12 era architecture as a minimal implementation level. That could be an A12X ( or A12Z since exactly the same die) or something further down the line.
With AppleTV+ rolling out on AndroidTV streaming devices in 2021 ( and embedded already on some AndroidTVs) , on RokU and FireTV , gaming consoles , etc. a lower priced AppleTV probably isn't going to do much for folks that solely just want to use it as a streamer. Apple could make AppleTV better at being a HomeKit server (secure video processor , etc ) . A more affordable gaming console. And some other value add things. For most of those the A12 doesn't lend much deep leverage to relative to the alternatives.