If you have a X470 or X370 and you were just going to drop in a new CPU I wouldn't buy a new X570 or a B series board but I would wait until the reviews come out.
You know there's going to be a lot of independent testing where they compare the new Motherboards to existing AM4 boards. I'm sure there's going to be more than just PCIE 4.0
advantages if you get the X570. I have a X370 MB and I'll wait till the pros and cons are weighed before I decide whether I want to buy a new Motherboard.