Re: "will need updating"
"There is nothing to suggest that pre Oreo devices couldn't be patched"
No one is saying that it's impossible to patch, in fact, that's part of the problem, we know from experience that most Android phones don't get updates for more than a year or so after they're first released.
Personally I'm willing to put up with the occasional flakyness of a custom ROM (Lineage), and so will probably receive a patch for this in the next week or two, but the majority of Android devices out there will never get patched. Despite the fact, as you point out, it's possible to patch older versions of Android.
Oh, and why? Because there's no money in updating an old phone, when you could be selling people new phones.