So one thing I notice... Orthstirr is finite. The more viking cultivators there are, the less of it there is to go around.
...and we know that the Orthstirr boon came from a sacrifice of some sort by a powerful cultivator. I'm guessing... basically, that sacrifice created a certain amount of orthstirr, that just keeps moving around in accordance with what the guy who created it considered good behavior.
By extension, if we just induct a bunch of people into Viking Cultivation, the orthstirr will get thinner and thinner, but it should be possible to crank up the total through further major sacrifices by high-end cultivators. Would likely be possible for someone who did that to adjust what was being rewarded as well (at least by having their patch of generated orthstirr reward somewhat different things).