Well I don't have a rule, but one seller with a couple a year doesn't seem rare to me, if it wasn't a collected poster, nobody would notice.
I wouldn't say a poster where even just 100 exist is rare, are Mondos rare? And modern posters as we know aren't printed low (except maybe those lenticulars).
For me it would be a couple or few known to exist. Or maybe better, if you want it this week, say the matrix poster, can you get it (forget price)?
Or if you want something, and you have to think, I'll try and find it in the next ten years.
If I wanted a matrix dreadful lightening, pulp fiction or ferris, it would be easy to get, you just email mopo, message here etc and offer a price around highest sales price and someone would sell it.
On the flip side, I spent better than a decade looking for ANY paper on the film TO THE PUBLIC DANGER. only was one example of anything online, years of asking mopo, and other forums, nothing, except this one poster.
I now have it. So, if I asked the same, forget price, find me another poster for this film in a week, month, year, I have doubts anyone would be able to. (Like to be proven wrong)
And because it's not sought after, I could list it on eBay and probably get no bids.
And yes you are spot on about older films and their lesser desirability, I guess that's my point, it's not about rarity at all. But it's about a bigger demand than supply, then rarity is attached. SCARCE might be a better term for things that only come up a couple of times a year.
Anyway just thinking out loud really. And I don't mind if that's what people want, or how they collect, go for it, but I am trying to understand it.