This is actually a really bad comparison. Not just for the reasons other people mention (it require open walls, etc), but because you miss another serious iss

This is actually a really bad comparison. Not just for the reasons other people mention (it require open walls, etc), but because you miss another serious issue: Fiberglass insulation is, by and large, installed very badly. Leaving even small gaps between the stud and batt (as most installers do), or compressing it too much (as most installers do) loses not an incremental amount, but instead almost 50% of the insulation value. This is why open cell spray foam, even at the same r-value, does so much of a better job - it's less prone to installation error. In any case, people often do not get the expected value from insulation, due to installation error. It's like people who spend 15k on a new HVAC system, and then have them install the whole thing using flex duct. People don't know enough to know what "right" looks like. By comparison, Nest requires almost no brains to install and use properly.