Most of the rumors of a new ATV claims that it will have an A12 in it. That is hardly an upgrade to the A10X in the current ATV4K.
If the new ATV does have an A12 in it, I suspect the reasoning of doing this is to lower the price point of the ATV, or raise the profit margin if the price stays the same. This would be from economies of scale from the A12 vs the A10X.
Currently, the only device that Apple sells with an A10X is the ATV4K. Apple still sells multiple devices with the A12.
If Apple wanted to give a new Apple TV a performance boost, it would have the A12X or A12Z in it.
If you purchased an ATV4K today, it isn't like it would be obsolete as soon as a new ATV comes out, especially if the rumors are true, and it has an A12. I guess there could be other HW advantages to a newer ATV, such as newer HDMI standard, new antennas, etc. but it is not like the ATV4K would be obsolete any time soon.