No joke, I cannot think of a better 'starter' CPU. The single threaded performance is more than adequate, the price is really nice and they can get a 12-16 core CPU for the platform if they ever need that much power.
I would have probably gotten a decent B450/X470 board before getting an X570 (if budget is important; IIRC the X570 boards were a decent bit more expensive). There have been a couple reviews from TPU(?) showing that even a lowish end B350 board can handle the 3900x at stock.
However, if you have a X570 board and it wasn't too much more, by all means, keep it. Nothing wrong with a bit more future proofing (PCIe 4.0 and any other extras they have).