I have been using 2 North Face bags since the early 80's, they loft fine, 20 degree bags still keep us warm in given temps.
Synthetics can loose 30 percent pretty easily in a year or two with regular usage. Compression is the killer. Regular usage does not mean how a guide would use it either.
Down can loose about 10 percent max.
Regarding bags or clothes, I have never really had a down piece loose much, but I have had synthetics that were say warm to 25 as glassing clothing, turn into warm at 40 after 3 years of regular usage.
If memory serves me correct, some of the older / heavier synthetics perform better than the lighter ones.
For me, I use down .. I may use synthetic to augment and move moisture in real cold weather.
As for clothes, I prefer down for warmth, apex for active insulation. I went downhill skiing the other day on a cold day where it was likely close to 0 F at tree line wearing Sitka Core Heavyweight, Arcteryx Kyanite Hoody, Lightweight Down vest, and a non insulated shell. I can do almost anything with some variation of that combo, add in sitka apex stuff for a lot of activity and a lighter shell when needed or a big puffy belay style for non active.