[...] because RAs are broadcasts sent by routers (plural, potentially) to announce network layout.
So? dns is a common part of modern network infrastructure. It's also possible you want to have devices use a dns according to which router they use (potentially, because the dns server may be on the other side of the router). Again, if they think dns in RA is bad design, it's still a common standard, it's not up to them to say "screw you" to those users.
That doesn't match up with the requirements for host config parameters, where you need a single authoritative source and you need the ability to receive machine IDs from clients so you can give out per-machine config settings.
A requirement that you just made up? A network can function properly with different hosts using different dns servers, there's no requirement to use the same.
Sure, RA doesn't have support for "per-machine config settings", but this is only something corporate environments need (and they can use dhcpv6). That just justifies the usage of dhcpv6 in those scenarios. It doesn't justify discarting RA completely for every user.
(Of course we haven't really stuck with that logic, since people argued that they didn't want to run dhcpv6 just for dns, so dns info was added to RAs. Then other people argued they didn't want to run dhcpv6 just for dns search domains, so that was added too. Where does it stop, I wonder...)
dns search domain are part of the network layout (just higher layer that IP/gateway). It makes perfect sense to include it into a single protocol that advertises network layout.
Again, none of these arguments justify not supporting RA. And saying that windows "fully supports IPv6" is mistaken, it only supports a certain network configuration. Not the most popular one by the way.