Think about those who bought a Ryzen 5 3600/3600X (because it was a great value, and they planned to upgrade when something substantially better is available) along with a solid X570 or X470 board (because they specifically wanted something with a great upgrade path).
That said, AMD and partners absolutely should make older boards (at least X470, preferably B450 too) compatible with the last Zen 3 as a lot of potential buyers have those.
If you are buying value parts like 3600 or 3600X, then you should understand budgeting and understand that it takes time for CPU to pay off and thus you are simply not interested in upgrading every second or third gen and you keep using that 3600(X) for minimum of 6 years and basically don't give a shit about 30% gains even. You only upgrade when you get at the very least 2 times more IPC, perhaps more cores and (most importantly) at the same or slightly different price. If you upgrade more often, you get less value out of value parts and that's counter productive if you are interested in saving some cash. Also nowadays CPUs, in terms of performance, last quite a while. After 5 years your chip might still be able to push more than 60 fps in games. You also realize, that in your library, there are likely many older titles or really old titles, so even if your chip isn't able to run latest AAA games at 60 or 50 fps average, but does 40-30 fps, you still hold up from upgrading as latest AAA games make up just a tiny fraction of your game library. It might take literally decade for that to happen (2600K, 3770K, FX 83xx owners would agree). The whole point of chips like 3600(X) is that they last a long time while performing well and they are relatively cheap to buy. But if you don't understand that and upgrade more often than you should, then value proposition just isn't there. Therefore, these minor updates to Zen chip line up on dead end platform just make no sense for nearly anyone. Perhaps they make sense for Ryzen 5 1400/1600 owners, but their boards won't support them. And if you look at things rationally, there's just no real value proposition in those chips. The only reason why they exist is perhaps some PR, artificially inflating AM4 chip portfolio and more than anything satisfying OEM demands, that put them in Dells, HPs and other machines. Maybe AMD made more of them than they really need and thus will be offloading them to customers and will try to market them as value purchases, when they really aren't in most cases.