You have some valid points but Apple isn't going to offer an extra 8GB RAM because the chips were "ready to just pop in". It's an embarrassment to Apple that its Pro tablet (which can cost up to $2400) can't run its Pro apps. Apple makes video editing software but yet they have to demo a third parties product at their hardware launches... embarrassing. At the very least the 16GB option I think almost guarantees that Final Cut Pro is coming to iPad Pro - those 4K video files will need that RAM and yes it's a small sliver of the iPad users who need that but they are paying dearly for the capability since iPads with 16GB cost more than MacBooks.
Well yes, hence my last sentence "That doesn't mean pro apps aren't coming that need 8gb." I think that is a real possibility.
But the "16gb meaning MacOs is coming or some radical OS re-design" is LOL. 99.9% or more of iPads sold and currently out there are on 8gb or less that Apple has to work around with the OS.
We are talking about a niche config of a niche product here. The most sold iPads surely are the base or Air by far over any Pro; by a mile likely too.