Well... if Avgas had left out the (paraphrased) "just use cheap stuff for a million miles and stop wasting your time" blather it would not have been a too in your face challenge.
It would be "show me what I do not see".
Avgas: There can and will be unicorns of experience. You're correct that many engines can "manage" to get by without adhering to OEM spec's. On many cars, OEM spec's are the best "guestimate" at what will work over a given large sampling of your user base. There you get the Goldilocks factor where some will be short ..some will be long ..but most will be just right.
Now with the introduction of the OLM (I hold GM's the all time best invention since sliced bread) you DO INDEED see some folks managing 12k-13k OCI's on something as basic (not to be regarded as cheap) Pennzoil conventional.
UOA, for many here, just reinforces their notions of how well their personal habits are bearing out in fact.
..and as far as your reference to the Euro's. I'd love to see some used oil analysis following the OEM recommendations before they got on board oil factoring implemented. The engine may have made it through all the projected years of ownership without issue, but that can just be a durability factor.
As a respected friend once said to me (paraphrased), the way you can play so far outside the margins with lubricants is more a testimony to the robust design of the engine than it is the suitability (or use) of the lubricant.