Yes if they did that they would be doing the same crappy thing that Intel do for the same crappy reasons and deserve the same criticism. Lets wait and see, we don't even know if Zen 2 will be refreshed yet, its just a rumour.
Anyway... this is a good point, a lot of these older boards cannot support Zen 3, others can disagree i think that's right, if they had a mix of older boards that could and couldn't that will create confusion, people will buy old boards thinking they will work with Zen 3 and they don't.
AMD will get the blame and the bad press for that, that's the reality, so AMD are covering their arses and i don't blame them. Frankly. 3 years ago AMD were not in a potion to make demands of board vendors to bake in 4 generation compatibility, they probably had a hard time convincing them to invest in them at all...
https://youtu.be/PzrnCniI7IM?t=4281