The 64 and 128Gb is just storage doubling for the current device. Something that would be relatively easy to predict for a streaming box that only needs low storage to load simple streaming apps and store a streaming data cache. If Apple were continuing with an A12 powered straight successor to the 5th generation AppleTV they can easily do this - and if they were to go back to the aluminium mark 3 remote it could be even cheaper.
If you're talking about big games titles with loads of data to install Apple will have to go with a 1Tb storage minimum. This could be the reasoning before going for a 'Pro' model with A14x CPU.
Apple simply can't assume that people will have adequate speed internet plus local storage to be loading data for a game every time a player wants to play it. Yes there's the concept of streaming games - where level assets are downloaded on the fly - or perhaps games could be properly streamed from a server using the AppleTV as a thin client.
In addition Apple's pricing scheme can't work there because if you assume that the A14x is on the cards for a 'pro' gamer machine then Apple can't be pricing a 1Tb model at $999 because it simply wouldn't sell. They'd have to go all in with the console way of doing things - keep the margins razor thin to get a bigger user base and rely more on software sales to make your money. Apple would be right on board with putting the specs into the console and then keeping it consistent for 5 years though.
Now, if Apple decide they want to do this they could sell an AppleTV Pro with A14x (8Gb RAM for the sake of argument) and 1Tb SSD storage for $399-$499 but require an Apple Arcade subscription to do anything other than Airplay to it. Equally, the box would also not work for watching TV/movie apps unless the user had an active AppleTV+ subscription.
Obviously, this will lead a minority of people to try and jailbreak it to run macOS ARM in the future but that's a bridge that could be burned with an Apple DRM solution.
