The G is a 4 passenger luxury sedan/coupe meant to compete with the 3 series, the Z is a 2 passenger halo/marque sports car akin to the corvette. You're simply confused because they happen to share the same FM platform, along with other dissimilar cars like the FX, which is still cross-shopped more with the G and vice versa than Z.The G was absolutely billed, constructed, and sold as a luxury coupe based upon the Z. This fact is not lost upon other posters in this thread. I'm sorry you seem unable to grasp this.
Around the same time? Which car used it first? Yeah, that's what I thought.....
Yes, both were introduced in 2012 (one mid and one late iirc), not "used exclusively on a high end vehicle for at least a generation". If you're still confused here, a "generation" in car terms is on order of several years, and a few months is much less than "at least [several years]".
Just because VW has accelerated the process doesn't mean that, historically, there aren't numerous examples of platforms and tech moving from high end to lower tier. I suppose the time is here when all vehicle platforms revolve around the same core parts like VW is doing but it didn't always used to be like that. Even so, the manufacturers will always reserve the best and newest tech for their flagship models. To do otherwise would be ludicrous. The whole point of this discussion is to evaluate the merits of their efforts.
So have you actually cross shopped an A3 and a Golf/GTI? What makes one a better choice than the other? Leave my posts alone and try contributing something.
The point of manufacturers developing shared platforms is cost savings throughout the lineup, not as a matter of "tech". Since this is more confusing than years > months, I'll elaborate a bit further. This means they can make a "basic" car for let's say $20k, and add more complex/expensive features on top to also make a $30k, or $40k model, which might not even be in the same segment. Usually those more expensive cars sell less, and this is a great way to amortize their development costs onto more mass market models. That's why for example the Z is actually a somewhat compromised sports car, but not in a way that matters much to the target audience. Ergo, it has nothing to do with "trickling down" the good stuff that you're thinking of, but rather a way to make "special" cars more affordable/profitable.
Finally, instead of considering this as some throwdown, think of it as an opportunity to learn something from someone who knows what they're talking about. Becoming more informed is a benefit to yourself, so don't fight it too much.