We know that the gronti stop working eventually, but not why. It could just as easily be due to Slann geomancy as anything else, and yes we probably won't know unless we research it a great deal.
We do know why.
During the War of Vengeance, the "old magic" that had empowered the Rune Golems had begun to sink deeper into the earth, out of the reach of any Runelord.[1a]
I assume what you mean is "We don't know why the magic sank." which is true... but I've always been arguing that assuming it hasn't been derailed until we have evidence it has been is the simplest answer, and until we can actually research all those other unknowns, the best we can do.
It's also just not relevant for the quest for two reasons. 1) Its incredibly, absurdly far away, long past our reasonable play length - this quest will very likely end before that happens. 2) Khazagar and Barak Azamar may have largely ameliorated or mitigated the issue in the North.
Spending time worrying about a far future canon event is kinda a waste of time in my perspective.
Honestly I its probably closer than researching a way to create a to create genuine life. So relevant in response to the question. Or the question itself was at fault.
2) Or by drawing it up faster we might have accelerated the drain, or just accelerated it elsewhere in the empire. Or the Deep Magic is coming from a Geomancy Wayline and once it snaps back it won't matter how deep we try and drain because the leyline will be moved entirely. Or the Deep Magic is from a Wayline, but the increased draw of BA/Khazagar will allow us to keep pumping until Maz shows up to see who's stealing his magic juice. Or a huge number of other possibilities.
Yeah theres a lot of unknowns, we all know that. What I'm interested in is what represents the fewest assumptions of those unknowns.
With the waystones out of the image, (shame about it to me, I like the divided loyalities interpretation of the waystones for the dwarves, made the most sense to me)
If it makes you feel better, we are predating the sort of elven collaboration that made that possible in Divided Loyalties timeline, so its still possible that something like that will be a Khazor like wonder we can contribute to that will come out of the Waystone research.
... Although I'm not sure if the dwarvern megaproject that it powered in DL were literally created by the Ancestors, if so the waystone network in DL must have been created earlier than now to power them. Some things like the runeworks keeping thundermountain under control seem to have equivilants.
Deep magic is the unaspected magic that runs through the geomantic web of the old ones. It's whats drawn forth from the polar gates that were ripped asunder and lead to the rise of magic in the world until the vortex was created.
What I don't like about the geomantic theory is that the KA stretched North to South from near artic to the equator, and east to west until the coast, if its a web drawing it somewhere, there should be somewhere in the KA where the deep magic is still flowing through and can be accessed.
