What I notice is that DHCP has become very heavily loaded with options.
Although one can deploy DHCP without using all those options, it has
become a complex thing. SLAAC at least has the advantage that it's
stayed simple; in fact the only overloading so far is RFC 5006. I think
we'd need a very convincing argument to depart from that simplicity,
as opposed to simply saying: if you need to convey arbitrary parameters
to hosts, use DHCPv6, which is intended for that purpose.
A DNS server address is not an arbitrary parameter; along with a default
router, it's the second address you *must* know in every host.