You can always buy an AM4 board and pair it with a less expensive Ryzen proc and upgrade to the 3900X (or whichever suits you) later without having to replace the motherboard.
Yup! And that's what I did for my first PC build: an R3 1200 over 2 years ago, recently upgraded to an R5 1600AF. Eh, not up to the level of an R9 3900X quite yet but nice to know the option is there.