Those who are saying they will stick with a mileage whether using Premium Gold OR an EC-1 rated ELC are BOTH missing the critical components present in any fluid duty cycle namely contamination threshold or neutralization capacity.
For engine oil this is easy. Consider contamination threshold: 20 micron filtration (stock filter) is not as good as 2 micron filtration (oil bypass filtration) but this is not the complete story as eventually the filter fills up with contaminants; it is obstructed. Now consider neutralization capacity: TBN above 1.0 means there is sufficient additive to prevent acid induced corrosion but there is also shear resistance (neutralizing friction over a period of time...perhaps neutralization is not the most accurate way to describe this but shear stability is a measure of the engine oil to provide consistent lubrication over a period of time during conditions that cause a chemical breakdown...sort of like acid/base neutralization - sort of). In any case, with used oil analysis (UOA) there is no doubt that when wear metal concentration rises it is an indication of the oil being less effective. When UOA shows falling TBN it indicates the oil has less capacity to prevent corrosion. When UOA shows viscosity reduction it indicates less protection to moving parts at operational temperature. There is an accepted duty cycle for engine oil, recommended by the manufacturer, of 5k to 7.5k miles BUT with UOA you may have to make adjustments lower (some may go higher although I haven't seen many oils that can handle the shear induced by the 6.0L for longer than 7.5k-miles, TBN and contaminants are no problem for most especially those with bypass filtration).
The same considerations ought to be applied to coolant whether Premium Gold or EC-1 rated ELCs (of which there are many). Premium Gold (which is technically an ELC and causes quite a few folks some confusion) begins with VERY LOW nitrite level...forget about silicates for a moment although those silicates should not be ignored...nitrites are a corrosion inhibitor. Corrosion induced by cavitation, AKA erosion corrosion, will literally eat an engine bit by bit all with microscopic bubbles. Corrosion in large part is a result of oxidation. See this link:
Water Purification Handbook Chapter 24 Corrosion Control-Cooling Systems
Water has an oxidation state of one. Nitrites have an oxidation state of two. As I mentioned, Premium Gold starts with low nitrite concentration (I think 800ppm) yet the minimum threshold in industry is 800ppm! Nitrites are depleted rapidly through chemical reaction. See here:
Learning Coolant Fundamentals
In an EC-1 rated ELC, carboxylates are introduced to provide cavitation protection extending the life of nitrites. Carboxylates have an oxidation state of three. Additionally, carboxylates add corrosion inhibitors for steel while nitrites only inhibit corrosion in iron. Most EC-1 ELCs also start with a nitrite level well above 800ppm.
The bottom line is that without periodic testing and analysis - an interval based on mileage is problematic and any desirable attributes such as corrosion protection or contamination avoidance is absent potentially and a false sense of security provided. That is the fundamental flaw of "fill-and-forget" in practice.
For what it's worth...the need for the discussion is still relevant from my perspective.