Buying mature vintages is an entirely different approach. It may make more sense for you, or less, depending on many factors. But if you are mostly buying mature wine that is ready to drink now or over the next handful of years, then you’re not asking the questions raised by this thread. If that’s how you buy wine, then there’s no age at which you should stop buying - until you shuffle off this mortal vineyard or reach the point where you decide to stop drinking. And if that’s how you buy wine, the answer to “how much should I have on hand” is going to be maybe two or three years’ worth (again, depending on many factors).
As David said, it also obviously has to vary a bit here and there. With some wines I will just buy the same amount each and every year. With others, like Bdx, I’ll buy more in better (or better priced) vintages and less in others, so we are talking averages here. But when you spread it over 30 years, as I do with Bdx, averages are pretty easy. And yes, guessing 30 years in advance the age at which I will stop drinking is obviously nothing more than an optimistic guess. I’ve decided to err on the side of being more likely to stop drinking with wine left in the cellar than to run out of mature wines in the cellar before I’m done drinking, but everyone has to decide on their own approach.
I also agree that this is a pretty clinical approach. I try to follow it anyway - not because I prefer a clinical approach to a hedonistic hobby, because I don’t, but because I find a clinical approach necessary because of the intersection of my hedonistic hobby with the decidedly non-hedonistic realities of budgeting my spending and my storage space. If I don’t take an approach like this, then I run out of money or space before I run out of wines I want to buy. So I have to be fairly analytical about how I’m going to allocate the limited resources of money and space across the far greater number of bottles I would buy if money and space were infinite and hedonism drove all buying decisions.
That being said, the clinical approach is, at times, far more aspirational than reality, but I do try.
The one knock on this approach that I disagree with the most is the idea that it is unrealistic to stop buying a particular category when you hit a certain age. Granted, I have not always been able to limit my buying to my budget, but the call of “you can afford one more” when you’re still young enough to see the bottle to maturity is different than when you’re not. Since I stopped buying vintage port, I’ve had no problems ignoring offers for more, and I can’t see myself in my 75th or 80th June getting tempted by en primeur, unless maybe it is a great-grandchild’s birth year or something. (Again, this only applies to those who prefer to buy young and cellar, not to those who prefer to buy mature wines at auction). I also think that the “Lay’s potato chips” principle applies here - mentally, it’s easier to buy none of something (especially if the reason is that you already have a lifetime supply) than to just buy one or two or any other small quantity. Once you take that step to buy that first one, it gets easier and easier to buy “just one more.”
Of course, my 75th June is a long way away, so I have plenty of time to change my mind.