@p >I wonder why DHCPv6 exists.
Since people can't help but implement broken degeneracy for IPv6.

I always disable DHCPv6.

>Does SLAAC tell the PXE where to tftp a kernel from?
No, as that insecure method of loading up proprietary software is a bad idea.

>If IPv4 doesn't work, your network is broken.
IPv6 just works nearly immediately, while IPv4 takes a while to maybe work, as SLAAC is much faster than DHCP and sometimes IPv4 breaks, but things work fine for a while as IPv6 continues to work.