In an engine, normal environmental degradation is not the issue. All oils (and viscosity modifiers) breakdown under shear stresses, where the molecules are torn apart over time. 100% pure synthetic Class IV or V oils are much more resistant to shear and thus can last much longer than conventional oils, but not forever.
Because of the higher shear stability of Class IV and V oils, they don't need to rely on viscosity modifiers. However, as @texases mentioned, many other additives are used, probably in higher concentrations than in conventional oils, to provide extended life. MPT claims their oil can go up to 30k miles between changes (with regular filter changes) under normal conditions, 15k miles under more severe conditions.
People wanting to extend synthetic oil use to the max can use add-on bypass filter systems to further filter the oil, down to 2 microns or so. Amsoil claims up to 80k miles (or up to 1200 hours or 2 years) using certain of their bypass filters. However, oil analysis would definitely be required in this case.