I think the good analogy for paper weights is t-shirt sizes: S, M, L etc.
I'd love to agree with you on that, but I honestly can't. I love (some, although not all) Under Armour apparel, but even within the same brand and just for tops — ergo, for which the same sizing chart is given by the manufacturer — I have to get different sizes in one style versus the next to fit. I mostly wear Medium in the brand's range (including both compression, 'athletic fit' and loose tops), but in some styles I have to get Small (or I'd be swimming in it; and, yes, I've tried the Medium one in a bricks-and-mortar retail store), and others I have to get Large, for the garments to fit my body properly all on the same day.
It's a good enough ball-park that it's not unreasonable to start there, but it doesn't mean it will fit, be comfortable, etc (wait, you wanted breathable, durable and water repellent?)
If one's requirements (for a particular application?) is multi-faceted, then it makes sense that the paper weight metric only addresses (at most?) one facet.
Like t-shirt sizes, the weight tells you little. "Will 70gsm paper be suitable?" is just as under-specified a question as "Will 20lb paper be suitable?".
That said, the g/m² metric is very helpful when I was trying to estimate how much a sheet would weigh
for mailing purposes, or how much 4,000 sheets of Tomoe River 68gsm paper would weigh and hence cost me to ship from Japan if I were to place an online order.
The problem is when consumers/people look for "perfect fit" with their discretionary spending, but most of the time they aren't even consciously aware, much less are able to articulate, all their requirements for one or more applications to which they want to put a product or service. I personally don't think the idea/objective of published standards is for prospective users to get what is "perfect" or even "suitable", but simply to allow them to individually perform gap analysis against what they need or want, and decide the impact of accepting a given compromise.
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