Searching for an early, highly theoretical, even philosophical, math paper on models or small-world networks
Stanley Milgram, The Small World Problem, Psychology Today 2, 60 (1967)
seems to fit the bill: +50 years old, "kind of philosophical", and yes, iconic -- cited more than 9,000 times. There are a few related papers in that time frame, listed here, but the paper from Psychology Today had the largest impact.
I am answering my own question, because I actually did find it again (after 5 years and many tries). It was not even close to being iconic nor +50y (it just looked old) nor specific on small networks :)
Rosen, R. (1993). On models and modeling. Applied mathematics and computation, 56(2-3), 359-372. PDF
Thanks @Carlo Beenakker for bringing up the Milgram paper!