Everyone slates Intel for the chipset change, but looking at the recent AMD release I get the feeling most who are moving from e.g 1700 to 2700 are also moving from X370 to X470.
To be fair, B350 and X370 have their share of issues, i.e. memory incompatibility. X370 is the better of the two, but from what I've read online, X470 is better than either. There is a lot on the IMC as well, but the chipset definitely bears some of the blame and this being an enthusiast forum, it's basically a shortlist of people who are most likely to pay the cost to get the last scrap of performance. There's also questions around VRMs, which on some B350 boards are really working hard and hot even with Ryzen 1 - and the new ones are more power hungry so...
On the other hand, stuff does WORK in older boards, just perhaps not as well. Hopefully the 400 series chipsets will be better built, and we're already seeing some overkill VRM options (Asus CH7 holy cow) that should be good for Ryzen 3