Collisteru's Shortform

You won't keep consuming the exact thing forever, you don't have plenty of space, even non-perishables can be damaged or degrade in long-term storage, and you don't want to expend organizational and indexing capacity on something you can just buy later.

For personal-use commodities, the optimal amount is generally whatever unit Costco offers, or the duration of your cost/convenience indifference point for thinking about it.  I know of zero cases where that's more than a year for me, and of zero where it's more than 3 years for anyone I talk with.

Actually, I may have over a years' worth of some light bulbs and some battery sizes.  It's insane to propose "lifetime", though - these things will get somewhat better or cheaper (relative to my income) over time, and I can't really predict any changes in size or preference changes over more than a few years.

I'd love to see the calculation / estimate of savings for some things you prefer to pre-pay and then store at your cost and risk for more than a few years, rather than waiting to buy closer to when needed.  

Things that I buy in bulk once I have settled on a satisfactory option:

• pens, whiteboard marker 
• writing pads
• sticky tape and poster strips (I love these)
• tooth paste
• soap, shower gel, shampoo
• socks, underwear

But typically not more than a year's worth or 100€ per bulk.

That is a great point. I would like to add:

'The optimal' X is not static. (Even if there's only one.)

The optimal supply of some items - including toilet paper, and probably water - has changed in my lifetime.

In my experience the reasons you give against stockpiling, and several others, are invariably overwhelming. You mention lack of space, change of technology, shelf life, and difficulty of moving. To these I add change of taste, discovery of new things that I like better than what I have, the mental burden of having so much stuff, the futile attempt to stop time instead of flexibly responding to whatever life brings, and that a lifetime supply could be very much larger than you're thinking given radical life extension or the singularity.

Instead my rule is: one in use and one spare. When the spare comes into use, I replace it at my next convenience. The numbers can vary (I have several pens in every room, a box of spares, and two in my pocket whenever I go out), but that is the general principle.

There are serious problems even stockpiling a decade worth of most of these items.

Most of the items in your list do actually expire. Many household cleaners degrade over time, rubber gloves perish, band-aids develop adhesive problems (such as becoming unrecoverably bonded to their packaging), and all of the things you listed in "if these never expire" do actually expire, except some pharmaceuticals. It is not easy to find out which ones, and if your findings are incorrect you could die. Bar soap isn't too bad, but some varieties can either deliquesce from humidity or crack and powder. Liquid soaps and shampoos can degrade in as little as a year. Toilet paper is bulky. For things like screws and nails, you can keep quite a stockpile but you will definitely not know how many of which types you're going to need 50 years in advance.

These are just some of the things I've noticed so far in my life without even trying to stockpile. I expect there are websites devoted to things like long-term fallout shelters and such that will have a lot more detail on the things that can go wrong with maintaining 20+ year supplies of everyday things.