THIS IS WORSE THAN PRODUCT SEGMENTATION.
Having multiple options is worse than only one option? :-o Over-dramatic much?
X470 and B450 are real options even for new AMD CPUs. Also, the lower tier chipsets will be made by ASMedia (X570 was by AMD), and will definitely be lower cost, so might not be that much better than something from the 400 series.
Since X570 and the motherboards using it are so over-built, have a multitude of new features, and all the motherboard vendors have put their best foot forward with some great designs (obviously a few duds too), there really is no need for "X670" this year other than that it would be "new". By serving two CPU generations with the 500 series chipsets, both AMD and its partners could save some on development costs for both hardware and BIOS. Maybe some refresh motherboards, but that could be it.
If that is the case, the B550 and A520 releases will probably coincide with the 4000 series APUs (based on the same Zen 2 architecture as the 3000 series desktop CPUs).