Okay so this may seem a bit of a leap, but hear me out. As near as I can tell, Groetus's big day is never going to come, for one simple reason. Monks of the Four Winds.
Let's start with Groetus's big day. According to what I've heard and read, Groetus is schedule to come when the last soul is judged, being sent to its resting Afterlife and all that, leaving the world empty. Basically he'll swoop in and pack everything up for the big Reset or whatever, but he's very specifically not supposed to come until the last Soul is Judged, right?
That brings me to my second point. From what I've heard the various Rez spells, including Reincarnate, are Pharasma deciding you're not up to be Judged yet, and sending your soul back to Golarion with her blessing. As such, as long as you are brought back by a Pharasma-approved (aka non-Necromantic) method, your soul is not yet Judged.
Okay, so let's look at the Level 20 Monk of the Four Winds. To start with, Immortality, they will not die of old age. Okay that's neat. Being monks, they also can't die from Poison or Disease, because they are immune to those effects. Neat. So the only other way I can think of that they could die is to be killed, right? But then there's the other clause of their Immortality, any time they are killed they are Reincarnated. This means, as I mentioned in my previous paragraph, that Pharasma is sending their soul back unjudged. And this kicks in every single time they die. Their soul basically will not, arguably even cannot be Judged, which means that from the moment the first Monk of the Four Winds survived to hit level 20 Groetus's condition was rendered un-achievable.
Reincarnation cycles are not infinite, even if the class itself doesn't state a limit. That's why Pharasma allows them.
Given sufficient belief in an alternative path, a soul can forego the River of Souls for a different fate, such as reincarnation. Such diversions are ultimately temporary, however, and at some point the soul attempts to reach the Boneyard.
