From what i remember, B450 and X570 were not going to receive support originally before the launch of Zen 3 , After a ton of backlash from folks online they thankfully changed their mind.
As for B350 and X370, support for Zen 3 happened around the time that Alder Lake came out, a full year after the release of Zen 3. At that point most of the folks that i knew that wanted to go up to Zen 3 but had X370/B350 motherboards ended up upgrading to X570 and later B550. At that time with Intel's not so strong 10th gen and the disastrous 11th gen before 12th gen alder lake, AMD were leading so like they could afford to do this.
I just hope that when they say support until 2027, they actually mean proper support and with no strings/bs attached that will result in someone having to buy a new motherboard down the road before the eventual socket change to AM6.