Based on what? The average update cycle time is about 3 years. The only time there was an upgrade less than 3 years was when there was something really missing from the current model such as 1080p and 4K.
There was almost a four year break from the ATV1 and the ATV2.
ATV2 had less than two years before the ATV3 launch, and I suspect the short period of time was due to the ATV2 not being 1080p, while the rest of the world, with the exception of iTunes, went 1080p.
The ATV3 had a new rev less than a year later, but that was technically a downgrade HW wise, as the first rev was dual-core A5 with one core turned off, and the rev2 had a single-core A5.
More than three years later, we got the ATV4 (HD).
But there was multiple issues with the ATV4 that warranted the ATV4K less than two years later, such as the relatively slow A8 and no 4K.
The ATV4K is still under the average upgrade cycle time, and it has a powerful A10X in it.
So, maybe long over due because of rumors, but not based off of history.