Womb Chair, Egg Chair, Swan Chair...etc.  

.
Oh my. Soil is tragic...worn (within reason) is acceptable in fabric furnishings, methinks. Maybe I'm wrong again.
But I'm odd,
Aunt Mark

Actually fastfwd,
I'm probably more comfortable sitting on slightly worn upholstered furnishings due to the fact that I tend to spill my beverage....always on the wrong piece...at the wrong time. My "splashed cocktail" addition to any thread-bare piece only add to the charm, as far as I'm concerned. Plus, the hostess doesn't notice.
Sad but true,
Aunt Mark.
ps I don't sit on anything velour, I might sweat, and absorb an odor.. I used to have issues with vinyl , but now I like vinyl. Yep.

DavidR
You said, "Imagine sacrificing a beautiful cognac leather egg chair because the foam has turned to dust."
I've never seen that happen. Did you read what I wrote about replacing foam while retaining the original leather? It can be done. It's just expensive to do. though not really more expensive than redoing the chair entirely with the additional cost of new leather. But if the original leather has a beautiful patina it can certainly be saved if you have a skilled craftsman to who can execute the restoration. It's just like fine art. Valuable paintings are restored and repaired by conservators every day. The artwork just has to be important enough and valuable enough to justify the expense.
In my experience, it has rarely been a close call and I've not seen a beautiful early vintage chair with lovely old leather discarded due to crunchy foam. It's more often a very clear, very easy decision. It's either fine as it is and you leave it be, or it's totally trashed and reupholstery is your only real option.
That is why early examples in a nicely worn (but not worn out) leather with foam that is not completely deteriorated bring a premium.

fastfwd,
Yes, I am mostly ...
fastfwd,
Yes, I am mostly speaking of leather. With something like an original and spectacular Girard fabric on an upholstered Eames shell chair, I have the same reaction. The foam goes and then there goes the fabric with it quite often. I don't think fabric gains any real patina, of course. It is very special to find a piece with original fabric still in nice condition.
Pegboard Modern,
I just read over your post again and wow! I'm pleased to hear that such a thing is possible. I also took a bit more time to think about your reference to the differences between breathable fabric and vinyl/leather. It makes a lot of sense that many vinyl and leather examples have held up well with regard to the foam. I have had my eye on a nicely worn egg chair, though was not sure about the condition of the foam. I suppose that if I find a leather chair with decent foam, it may still have plenty of life left! Now that you mention it, I actually sat in an early vinyl egg chair not too long ago and was surprised by how well the foam had held up. Lastly, thank you very much for your explanation and logic. Your wisdom is greatly appreciated.