My take is this, most x470 boards will run a 16 core just fine. A ridiculously cheap, and I mean really really cheap low end board might not but any x470 board is basically designed for overclocking and can easily push a 2700x up to using well over 150W with no issue.
Lately you have a bunch of guys, particularly youtubers making all these claims about bad VRMs, for the most part when they say VRMs are bad they are really saying, but not specifically so being stupid, that the one high end board with way WAY overspec VRMs has enough juice to push say 300W comfortably and the 'crap' overclocking board with the 'crap' VRM is only capable of pushing 270W... so it can't match the board that can push 300W via VRMs. In reality, at stock or a light overclock there isn't a high end overclocking board.
There are two issues here, do the manufacturer in AMD or Intel, allow the mobo makers to update bioses to support and validate the new chips or do they want to sell more chipsets and get a cut of motherboards, and then if they allow it, will lets say Asus decide to use that permission to update their boards which may lead to them losing a sale as someone flashes their x470 instead of buying a new x570.
Intel have made a huge deal about intentionally blocking older chipsets using newer chips which stops most motherboard makers enabling support because they risk the wrath of Intel for going against them. AMD usually go the other way and would be more likely to let their partners update their boards.... but the partners really don't want to. Really it becomes if one big manufacturer wants to win long term customers by doing the right thing and being seen as the reliable company, then the others will probably fall in and support them also.
Realistically it would also be VERY easy to simply say a 16 core chip will use 95W and lets say for arguments sake, 4.2Ghz all core boost on x470 but will enable 125W mode on x570 enabling 4.7Ghz all core boost. Allow that to be the 'stock' setting that can be easily validated on all x470 boards, but in reality that is only a stock mode to meet a specification, you can overclock the 16 core just like any other chip and either use the normal higher clock speeds or overclock even further.
Basically AMD have talked a lot about supporting older motherboards so I can't see them getting in the way.
Then on the 8/12/16 core. I think Intel aimed 10nm at a good couple of years ago and Zen 2 was supposed to come out quite a long while after Icelake. AMD were likely banking on 14nm Intel staying 4 core and Icelake a year or two ago maybe moving to 6 core, but being on a process that it would be easy to scale to 12 core or so without much trouble. AMD planned Zen 2 to go against Icelake, they've said so. It's why it's designed to be 16 core on desktop. The thing is Intel couldn't have missed 10nm/Icelake releases worse than they have. Even now every sign is desktop Icelake is going to be a fair way into 2020. This means AMD doesn't have to go 16 core asap to beat Intel, however with 8 core 9900k.... 8 core Zen 2 is a waste imo. 8 Core puts them on par with Intel, maybe slightly ahead and for maybe 6-12 months. If they release at min a 12 core, if not 16 core at launch then they absolutely kill Intel mainstream immediately and utterly dominate for a year at which point Intel will likely only catch up, or worst case won't even catch up.
So they could go the route of I think at worst, 12 core at launch, hold back 16 core to smack Intel with at a later date and milk the 12 core for a while as with the 1800x, then bring in the 16 core at that price and drop 12/8 core pricing at a later date. Personally I think they are going for the straight killer blow, straight 8-16 core product range available from launch and nothing Intel produce in desktop is remotely competitive till into 2020.