It put me in mind of something someone told me once, which is that in the business world, inventory is bad. Inventory is inefficient. Inventory costs you money in the long term. And it's the same with your personal inventory of yarn, as well.
The biggest objection to yarn stash is the liquidity argument. Cash is liquid: you can spend it right that instant. A car is less liquid, because you have to go to all the trouble to sell it before you can spend the money that it represents.
Yarn is not liquid. It represents a lot of your money, which you have locked up into fiber. Imagine if you had put all that money into a bank account, instead. You would be able to spend that money on yarn whenever you wanted it. But you could spend it on other things, too. (Like those sudden financial emergencies that seem to happen oh so regularly.)
A lot of people say "I'm buying yarn as my retirement fund." Not to be morbid, but what if you die before then? What if you suffer physical impairment and can no longer knit? What if you just lose interest in knitting? Wouldn't you rather be able to spend the money today?
Everything ages. Granted, yarn holds its value pretty well compared to other things you might buy. IN THEORY, a skein of yarn today is exactly the same as a skein of yarn twenty years from now.
In practice, most people find that they are not able to get "sticker price" when they resell their yarn. There are very few cases where you will be able to sell your yarn for more than you paid for it. In general, you can sell your yarn for about 80% what you paid for it.
That's a 20% depreciation! Ouch! If your money had been in a bank account, it would either have earned money, or remained the same amount.
Fire. Flood. Vermin. Mildew. Insects. Small children with scissors and unsupervised time. Bad fates can befall a yarn stash. Even a small house fire can easily smoke damage your entire stash beyond salvaging.
4. Storage Costs
Aside from the emotional cost (and credit) of your stash, there are very real costs just to hang onto it. You can easily calculate how much you are paying in rent or mortgage per square foot of your home. Then calculate how many square feet your stash occupies. That's how much you're paying ever month to store your stash.
It may not be very much money per month. But it adds up, month after month, year after year. It adds to the cost of the yarn, which means that 20% depreciation (see above) is only part of the story. Your yarn becomes more expensive the longer you keep it!