With the lack of B450s available in the current market, any new supply of decent motherboards is good. Personally I like the added features that come with the mid-range B550s. For under $200 you get 2 M.2s (1x PCI-E 4.0 / 1x PCI-E 3.0), 2,5 Gb/s LAN, PCI-E 4.0 GPU, Wifi and AC1220 audio. Added to this are VRMs that are more than capable to support the next gen of CPUs while also featuring the next-gen BIOS.
The only true limitation between a x570 and a B550 is the chipset itself. Less 4.0 lanes will not always be a limiting factor, but once cheaper 4.0 hardware rolls out this might be a factor for some people, looking to get the most out of a setup.
With the current CPU prices and the performance they deliver, it might be more reasonable to stay with a sub $200 CPU. Investing an extra $50 in a plattform that could support changes in interfaces for the next couple of generations of GPUs might be a better choice right now unless you run a RTX 2080ti+.
If software goes the way of more multithreading in the future, upgrading to a 4900x or similar will certainly be possible at any point. This could set you up CPU-wise for the next 4-6 years. Sure, DDR5 will come and so will PCI-E 5.0, but look at the number of users that still manage with DDR3 and a 4770K or similar on current GPUs. Processing-power is so high right now with $100 CPUs being able to pump out 4K content within reach of $500 10-core+ CPUs.
With the current market it makes little sense to go B450 unless you are on a strict budget route. For an extra $50 you pick up such a number of features that you would only get in mid-tier $200+ x570 boards. If you want to go low, at $120 you still get all the new interfaces with slightly less features. Sure, if you are looking below $100 there is little left, but I would assume a decent number of boards might breach that barrier in the next months.