Yeah, I would agree that unless you need the system within the next couple weeks or so, it's probably worth waiting a bit to see what the B550 boards have to offer. It's possible that B450 pricing might potentially come back down a bit as well once there is less demand for them after B550's launch. Right now, I suspect motherboard manufacturers have greatly reduced or eliminated B450 production as they prepare for B550, though it's hard to say whether supplies will recover after demand goes down.
Of course, a lot of this may depend on your overall budget too, and what other hardware you were planning to put in the system. What are the system's complete specs?