Richard, apply that logic to anything else in life...like a car,washing machine,socks...etc etc etc sooner or later stuff fails,brakes,gets worn out ,meets with a premature end etc,some stuff lasts longer than other stuff ,thats life . If we all didnt buy stuff that was one day going to fail no one would buy anything,fortunatly for the global economy few of us live our lives thinking like that.Life is short,enjoy the moment and dont dwell on what might happen to your little plastic guys, there are after all lots of more worthy subjects to worry about
Whilst much of what you say is true, and all of us expect that our collections will one day go the way of all flesh (although hopefully after we do), such occurrences are, nonetheless, disappointing. I wouldn't mind so much if i didn't paint the figures, or if i did a rudimentary paint job, but i would be gutted if those figures that i'd spent many hours painting to a decent standard were starting to crumble (i quick inspection revealed that mine are currently ok, must have been a later batch, but condolences to other people; as Justin Hawkins sings on 'Bald' "tonight, thank God it's him not me").
So far, the only issues with crumbling i've had in my collection is with some ancient airfix american marines (first edition) and some HK knock-offs from when i was a child. Until recently i hadn't made much effort to preserve my collection (of every manufacturer, from c 1980 (if not before for some that i acquired second hand) to the present) in optimal conditions, so i'm inclined to go with the bad mix hypothesis forwarded by gurus Mike and David.
To be honest, i don't think it would help for manufacturers to offer replacements of crumbled figures: their cost is tiny compared to the many hours of effort trimming off the mould lines (especially in the floppy plastic), painting them and basing them.
Yes, life is short, too short to put a lot of effort into ephemera. Perhaps hard styrene is the answer? Metal may not last forever, but i suspect my metal figures will see my plastics, and me, out, so perhaps i should expend my efforts on those?