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you're somewhat correct, but that's oversimplifying it a lot.
the more "multigrade" an oil is, the more potential it has for shearing down, but that doesn't mean that it will. this is the reason all of the hardcore BITOG guys do oil analyses when changing to a new oil, to see how far they can go on them before running into the viscosity changing too much.
also, 5 weight vs 5W or 40W vs 40 weight are very different. 5 "weight" is implying viscosity when warm, 5 "W" is implying viscosity when cold. just something to keep in mind.
as for your explanation of stuff being added, that's correct. the base oils are engineered to be very thin, then viscosity index improvers are added to slow down the rate of thinning when the oil gets heated up. this allows for you to have an oil that is thinner when cold and thicker when hot compared to a single-grade oil. the drawback is shearing will bring it closer to a single-grade oil as it ages. there are two opposing forces at work here though, shearing and mass loss. shearing takes the part of the oil that would make it thicker and degrades it's ability to keep the oil thick at higher temperatures. mass loss is the "phenomenon" that occurs where the lightest parts of the oil burn off at higher temps, causing the oil that doesn't burn off to be thicker.
the rate of shearing vs mass loss will determine what viscosity the oil will be at any one time. considering a lot of the BITOG guys go ~10,000 miles or more on an oil change, something has to be working well.
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