Re: "IPv6 was designed to run alongside IPv4 from the beginning. "
Sure, so well that DHCP was an afterthought - the RCF came only in 2003, five years later, - because they thought routers could assign addresses but didn't think about every other need DNS, dynamic IP-hostnames assignment
SLAAC could do all of those things. The reason DHCP6 happened was the exact inertia people are reporting here. SLAAC: It's new. It's scary. What does rtadvd stand for anyway? Brown alert!
So we ended up with two ways to do the same thing and now we have to support both on the gateway because the wonderful thing about standards is there's so many to choose from.