That's not quite true...for example, in a 20w50 oil, the "50" comes from measuring it's flow at 212 degrees F. The "20" is the flow when the oil is cold, as in when it is sitting in the oil pan in a cold engine with the ambient air temp also cold. It allows the engine to turn over more easily at colder temps. It is not a 20 weight oil any given temp other than when completely cold. So, with a 20W50 oil (or 20W60) you get all the benefits of better flow when the engine is cold for easier starting, easier intitial flow characteristics through the engine when first cranked, PLUS the benefits of a 50W oil when it gets hot. So, you can think of the "W" as the winter variable in 20W50...easier to pour (flow) out of a container at say 20 degrees F than would be a straight 50W oil. IMO, you get the best of all worlds by using a multivicosity oil...the advantages of both extremes in temperature.
You went into more detail than I, but we're on the same page as far as multiviscosity and the VI (Viscosity Index). To be correct and specific, viscosity always changes with temperature; therefore, to compare different oils, the SAE J300 standard (home of numbers like 20w50) mandates certain test temps. The table below shows the requirements to be classified as a certain viscosity. You can see the different grades have varying test temps (for comparison, -40 C = -40 F; -15 C = 5 F).
Column 1: SAE Oil Viscosity Grade
Col 2: Low Temperature °C Cranking Viscosity (mPa∙s) Max
Col 3: Low Temperature °C Pumping Viscosity (mPa∙s) Max (MRV)
Col 4: Viscosity at 100°C Min
0W 6200 @ -35 60000 @ -40 3.8
5W 6600 @ -30 60000 @ -35 3.8
10W 7000 @ -25 60000 @ -30 4.1
15W 7000 @ -20 60000 @ -25 5.6
20W 9500 @ -15 60000 @ -20 5.6