Twenty years is a long time even in development time. Mobil's "behind the scenes" testing to prove out new long oci formulations to marketing's challenge for a 20,000 mile oil would seem to indicate there is more to their claim than just marketing speak. Amsoil's claim for even longer intervals is backed by empirical testing as well.
When discussing oil change intervals, I don't think you'd disagree that the oil used determines how long it should be, using the oil manufacturers tested limits not the vehicles MM or a one size fits all 5k interval. The rapid degradation in only 200-300 miles is characteristic of one or more additives reaching their limit to fight off further oxidation, dilution, viscosity increase, acid neutralization, film strength, et al. I think you'll also agree that there are "hard" and "easy" mileage accumulation stresses on oil that have a huge impact on how often the oil should be changed.
The "old school" of every 3,000 miles with dino oil is slowly migrating to longer oci's as the better synthetics prove their ability to not only exceed 5,000 miles but go even longer. But not all synthetics are equal so perhaps 5,000 is a good conservative number for some, maybe many, while Mobil's 20,000 mile and Amsoil's 24,000 mile claims may hold up safely considerably longer than 5,000 miles by whatever margin your belief in the testing and risk aversion allow.
I'm comfortable going 8,000 miles or a year with Amsoil Signature and my use is severe ... short trips, stop and go, some towing. Never lost an engine using Amsoil and annual oil changes.