There are lots of things that bother me about inflation metrics and how they're discussed. You're getting at one of them with the constant utility thing: price inflation is an individual thing because indifference is a property of individuals.

Since preferences are unobservable, there's no plausible way to actually make these measurements correctly, so they do the most reasonable next-best things they can think of. Which brings me to one of my other pet-peeves: Economists are well aware of the problems with measuring inflation. It's not a nefarious and sinister enterprise. It's just an impossible ask that we've been charged with.