I want to point out that in the AM4 platform, the Chipset is merely a glorified PCIe Switch, SATA Controller and USB Controller. The Processor itself fulfills all the core roles as Zen was designed as a potentially standalone SoC. As such, the Chipset is an extra, thus it doesn't has to support the Processor at all, making it less important than what people thinks that it is. If anything, it is technically possible that Zen 3 works in any Chipset, or maybe even without (A la A300/X300, which were supposed to be just security chips wired to the Processor via simple SPI and not full fledged Chipsets wired via 4 PCIe Lanes).
What you have to focus here is in OFFICIAL SUPPORT. We know that AMD already said no, but that doesn't means that it will not work. If we go for that logic,
AMD officially does NOT support Zen 2 on B350/X370 Chipsets, yet they work regardless. Since there are no technical reasons for it to not work, then I expect that it should be possible to get Zen 3 working. Whenever the Motherboard manufacturers decides to support it on their own, or whenever you require some BIOS modding to force newer AGESAs onto older Motherboard (
As if that wasn't currently the case...), is something still yet to be seen and there is nothing else that you can do but wait because without physical Hardware no one can experiment solutions.
The thing here is that the Processor may want to validate the Chipset for certain features (Reason why A300/X300 were supposed to exist), and refuse to enable these features if not on supported Processor-Chipset combinations. Both Intel and AMD do that. However, on top of that, Intel uses Chipsets for market segmentation between Workstation/Server and consumer lines on the same Socket, making post-Skylake Xeons E3s to outright refuse to POST on consumer Chipsets. However, that is not a technical limitation, is a fully artificial one (And actually, it was defeated via BIOS modding, making such combinations now possible). The problem is whenever AMD decides to implement such checks in Zen 3 to not work in non-supported Chipsets instead of just crippling some problematic feature like PCIe 4 on non-X570 Motherboards.
About PCIe 4.0 on older Motherboards, that is something that was already well covered. While in theory you can, and early AGESA versions supporting Zen 2 could enable PCIe 4 capabilities (For the PCIe Slots wired to the Processor, obviously, the rest from the Chipset were still PCIe 2), PCIe 4 is quite strict and Motherboards not designed with it in mind would have signal quality issues (This seems to not have been the case with Intel Ivy Bridge, which introduced PCIe 3 support for the Processor PCIe Slots on older Sandy Bridge Motherboards). While Motherboard manufacturers could have come up with redesigned Motherboards based on old Chipsets where the Processor PCIe Slots were fully PCIe 4 compliant, AMD decided that it didn't wanted any blaming over stability issues caused by older Motherboards thus crippled PCIe 4 on all non-X570 Chipsets for that reason. I do believe that it could have been done, though, but maybe there wasn't any other way to discriminate between an old and new design based on a specific Chipset thus AMD prefered to blacklist them all to play it safe.
Also, I think that people are understimating SATA. For one, it still is much cheaper than NVMe SSDs, so you can't replace it if looking from a GB/$ perspective, and this matters for budget users (I suppose that there is a reason why there are budget Notebooks that still come with HDs!). Second, you can get far more SATA connectivity than NVMe, since NVMe is very heavy on PCIe Lane count, and to fanout PCIe, you require expensive PCIe Switches. Third, thanks to PCIe and SATA Controllers being multiplexed onto the same Pins in several chips (Including Zen itself and the AMD Chipsets), you can have fully expose both of them if a Motherboard designer decided to use an appropiated connector.
For example, X570 has 4 dedicated SATA, and 8 lanes that can be configured as either 4 SATA or 4 PCIe. If you used two OCuLink Ports, with cables you could get either a 4 lane PCIe NVMe drive or use
a breakout cable for 4 SATAs. That way you could maximize flexibility and let the user could choose whatever it wants (Albeit the cables are still a non-irrelevant cost). Sadly, they all took the pathetic M.2 route, which shouldn't have scaled up from Notebooks form factors to Desktop on the first place.