I have always assumed that the reason we can not achieve the same accuracy with cast (or at least it takes a lot more effort) is that cast bullets are inherently less consistent. There may be small voids, molds may not always close the same way, pouring consistency can cause differentials in fill, mold temperature changes (caused when the cadence is interrupted) affect fill,, alloy may be slightly different just after fluxing than after 50 pours etc etc.
If I understand the RPM theory, it essentially states, that there is a point where some "normal" variation in the bullet causes the bullet to become more unstable than acceptable. The bullet has always been unstable but stable enough to provide reasonable accuracy when velocity (spin rate) was behold this threshold. One this point is reached, the bullet must be "more perfect" - and/or it needs to be loaded with more consideration to the kinds of things we do for benchrest ammo- and/or the gun itself must be better (chamber consistency?)
We go ga-ga over a 3/4" cast group that would be relatively easy to accomplish with jacketed bullets in a hunting rifle. We know that something like a .35 Remington is more "cast friendly" (= less affected by bullet inconsistency) than a 5.56.
The loading techniques that allow us to achieve good accuracy with the more perfect jacketed bullets are not good enough to address the variations that occur in a cast bullet. So, we need to take more care to things like neck tension, bullet concentricity, bullet distance to the lands, etc etc.
This "threshold" will move upwards as we either load better bullets or improve how we manufacture the ammunition.
What I am not sure of - but suspect - is there could be a "harmonic zone". Much like a tire that is out of balance. It runs fine at less than 40 mph (even though out of balance), but shakes excessively at 50-60 mph, yet seems to run better again at 75 mph. When we develop a load we start from the sub 40 mph area and things start to go to hell as we get to the harmonic zone - so we improve the balance of the bullet/load. But we either do not or can not drive the bullet past this harmonic zone. Or it could be the imbalance is so destructive, the bullet can never fly true at any speed past X.
Cabin fever is a funny thing.
Don Verna