So let me get this straight this RX 5700 could be the spiritual predecessor to the AMD "70" Series (Aka 570) that normally built from the gelding of the full-mainstream silicon?
Sure Strange Brigade is a "ringer" and AMD architecture poster child, but also a good all around projection of the capabilities of Vulkcan/DX12 "API Overhead" and I'm sure why AMD leads with it. They're promoting the obvious in that there's gaming engines out there that can unleash their particular architecture design. Nothing wrong about that...
It is interesting that what's been AMD second tier mainstream offering, is working over (sure the one title) a part Nvidia has promoted more-or-less entry enthusiast while such AMD "70-Series" have been task as "entry mainstream" more a-kin today to the GTX 1660. If the RX 5700 actually is 20% behind a 2070 that still has it like somewhere between the Vega 56/64, and I would consider there still a RX 5800 (full-part) still out there.
I figured Computex was just the top-level design and architectural keynotes, and honestly not taking the "marketing jargon" or any of this at face value. That said, I think AMD has a good blueprint and executed this Navi release in fairly clear-cut strategy, while holding to the schedule (we'll wait to see how well they can fill the channel). They'll have more information to drop at E3 (June 13-11), but I don't think we'll learn a-lot with 3 weeks until the actual NDA release July 7th.
No, there is no AMD '70' series as you see it with Nvidia. AMD released
Polaris and then started incremental updates to that design, mostly in terms of more power > more perf. Polaris is designed as a midrange chip from the get-go.
Vega fed their upper half of the stack, development wise its disconnected from what happens with Polaris. Of course features exist on both product (lines) but the design is not the same; Vega has HBM, Polaris does not. Vega got other improvements, Polaris did not.
This also means your interpretation of the AMD naming scheme is not correct. Since the release of RX480, there have been new names but most of that has been rebranded or very minor improvements. Navi's new naming has no relation whatsoever to performance or place in the stack really, its just taking a look at Nvidia and slotting in on the right number. There will be a bigger Navi, but what it will do is a mystery, and AMD no longer has a structure you can rely on with their product stack. Gone are the HD-xx50 / xx70 days.
The key point being, we have no idea what the bigger chip will perform like.
With Nvidia, prior to Turing (but even now, really), while they do use multiple SKUs, these are all almost straight scaled versions of each other. Sometimes some trickery is applied (asymmetrical VRAM setup, usually found on midrange, not just GTX 970; Fermi and Kepler had them too) but you won't see a split halfway the stack using radically different tech. Turing is the exception with its RT components.