Actually, with a BIOS update, the new Zen 2 CPU's are indeed compatible with most x470, b450, and even 300 series motherboards.
AMD recently responded on reddit that they don't want to fragment the market so they just make zen 2 not compatible with old boards. That was after announcements by board manufacturers about compatibility on their boards.
Looks like I was for some reason confusing pcie 4 compatibility (which will not be supported even on top end mobos) with cpu compatibility. Can't edit the post though.
You can put new Zen 2 cpus into old 370/470 boards, except the weakest ones. Maybe they will change socket for Zen 3.