Is there a reason people say this? Why would down last longer than synthetic fill?
Synthetic bags have been a round for a long time, but down far longer. PHD has a very good article about the relative pros and cons of both
http://www.phdesigns.co.uk/the-truth-about-sleeping-bags, and while they make pretty much all their stuff with down, they fully admit there are times where the better tolerance towards damp might make synthetic more attractive. And I've been wondering a little about down, since I have asthma, and sometimes find down clothes a little itchy.
However, synthetics do tend to lose loft - my 22 (!) year old Snugpak Osprey 12 has been stored very carefully, but its not as lofty as it once was, and thus I'd probably downgrade it by a season. Thats fine, I've got my money's worth, but no matter how much I wash it and fluff it up, its never going to be the temp rating it once was (and it wasn't the temp rating it was supposed to be in the first place). If you store a synthetic bag in its stuff sack, then your going to kill the loft even quicker. Thats doesn't mean that something like Hollofill, etc cannot keep you warm for 30 years - it can. But its not exactly lightweight. But most synthetic fillings tend to lose loft, and at some point, you'll have to replace it.
On the other hand, down can be revived pretty well by just a decent wash (after 5 years, send yours to a professional company like these guys -
http://www.mountaineering-designs.co.uk/cgi-bin/trolleyed_public.cgi?action=showprod_CLEAN, and it will really fluff up), and will last much longer overall (figures seem to vary, but twice the useful service of a synthetic wouldn't be unfair, although 'last a lifetime' does tend to crop up a lot when looking through the net) than a synthetic bag. Its also
much more efficient as an insulator, as well as more compact and lighter, as someone pointed out a while back on Outdoor Magic:
for comparison purposes----got 2 winter s/bags ---both equally warm---down 1200gms --packs easily into 13 litre stuff sac---synthetic 2350 gms --struggle to get it into 30 litre stuff sac-
Back in 1994, when I bought my Snugpak, that bag was about £125 RRP (I got it for £87). A down bag in those days was at least £100 more expensive than that. So synthetic was pretty popular, especially once Snugpak brought out a bag which wasn't huge, etc.
The £100 gap is largely still true, when you look at a MH lamina and a Rab Ascent, but of course in real terms that gap is less, because in real terms, those prices have dropped a lot, thanks to Chinese production, the net, etc. That same Snugpak bag is still £125 RRP, but can be had for roughly £100. If the price had been the same in real terms, you'd be looking at twice that. So the gap has narrowed, and thanks to Alpkit, etc, the difference is perhaps £50. So buying a decent down bag then was much more pricey than it is now, hence one of the reasons for synthetic. Now, you can afford to go with down if you want, and DWR etc have mitigated some of the downsides of down filling to some extent. And while a MH laminia Z Torch is OK at roughly minus 15, it weighs 1.78kg and costs £155. Alpkit will do you the SkyeHigh 900, which is rated at minus 13, costs £200, and weighs 1.45kg. Thats roughly the same price, etc as a Rab Ascent 900 at Go Outdoors. For the extra £50, thats 300g saving and easier packing - long term, thats not a lot.
Down is more cost effective long term, and its easier to pack, etc. Synthetic is fine in lots of ways
https://backpackinglight.com/forums/topic/54025/, and I hear good things about MH, although I found the foot cut too narrow for me, but thats a personal thing. My kids are fine with Vango bags - cheap, warm, but have all the compression of a breeze block. Thats OK, but if I was carrying a bag for 20km on my back, down makes a lot more sense! The bulk of sleeping bags from well known high level brands (Rab, PHD, ME, Western Mountaineering, Marmot, TNF, etc) are all down - MH do a lot of synthetics, but their top bags are down. So the market does see down as suitable for both the UK and other countries, not just the Arctic, etc.
Buy once buy right is always good advice, especially when being warm and comfortable are essential. Spending good money on a good product, no matter what the technology, is seldom wasted. But ultimately, its whats needed, and whats best for that person.
