Yeah I think my purchasing advise is going to be on the lines of:
x570 top tier: Buy if you want NVMe 4.0 only or if you plan to do extreme overclocking
x570 mid tier: Buy if you want NVMe 4.0
x500 bot tier: Don't buy, even for NVMe 4.0.
x470 (any): Most people should buy a x470 and it will satisfy all their needs.
Justification: The top tier x570s will be using better routing, and better material stackups that the PCIe Gen 4.0 and high memory speeds require. You should find these boards to be the best for the latest and greatest of technologies to come before the next step in 2021/2022. Mid-tier boards may be able to handle PCIe Gen 4.0, but may not have the best routing, so you may see some data rate difference, but its hard to say what that difference will be until the boards are released. Material stackup at this tier will differ but its too early to say. The low end x500 tier is most likely not worth it, even if all you wanted was NVMe 4.0. I'm expecting the material stackup to start using older materials that may not completely support these faster data rates of PCIe Gen 4 and memory. The end devices may train but I expect much lower data rates than the top tier x570 boards. This means those in this budget range should just stick with the x400 boards for now.
On top of this, GPUs with PCIe Gen 4.0 are not worth it right now. Nvidia's wait on advancement is justifiable. However, AMD could gain traction in Gen 4.0 with multi-GPU setups. I think that crossfire and DX12 multi-GPU systems could take a lot of advantages with the faster data rates, but this will take time to mature. As I see it, there are no advantages for GPUs at Gen 4.0 right now.