I think the main advantages of the B550 could be better compatibility with Ryzen 4000 ...
Ahh...I will not hold my breath for that one! Absent a definitive statement from AMD, we can not be certain Ryzen 4000 will be deployed on AM4. Right now all we have from them is 'we'll support am4 to 2020'. Whatever that means I feel it speculative to infer 'with new processors' much less 'with new processor architectures'.
... two M.2 SSDs without reducing the second one to SATA 6Gb ...
THAT B550, with more PCIe lanes, should allow. And PCIe gen 3 too!
Incidentally, I do have two NVME SSD's in my B450m Mortar...the second is operating at PCIe x4 gen2 though. For that I gave up the second PCIe x16/4 slot which is utterly useless anyway. So it's not true the only way is with SATA 6 Gb/s bandwidth.
You don't have to do gen 4 to the GPU and NVME expensively, ASUS has demonstrated that already. The real expense comes with doing it to the other slots, over 6" from the source, that require PCIe repeaters or redrivers. The other big expenses for X570 was the AMD-sourced chipset itself and the heinously over-designed VRM's that the board mfr's felt needed for these boards.
B550 will be Asmedia, IIRC, design and mfr., in addition to foregoing gen 4 so we should get some good economy from that. And being slotted into the budget segment I hope mfr's can see fit to equipping them with VRM designs reasonable sized for the power efficient Ryzen 3000 product line.
One hopes, at least.