A handy thought experiment is to amortize an expen$ive down bag over ten years or more. It's relatively easy to get a decade or two out of a down bag and the cost per year really isn't bad. The problem of course is you can't set up a ten-year payment plan on a sleeping bag.

By contrast I've never had a synth bag retain its loft nearly that long. They seem to start going flat after about five years, despite my taking scrupulous care of them.

Cheers,