Wouldn't hedonic adjustments go the other direction from what the parent comment is saying? If the quality goes down, then the adjustment should increase the stated inflation.

I read the parent comment as talking about substitution effects in consumer behavior, but the CPI doesn't reweight month to month (it used to only adjust once every few years, but has recently switched to once a year).

So generally, substitution bias makes the CPI overstate the inflation as actually experienced by the typical household.