EUI64 means that random space is effectively reduced to just 48-bits of real entropy, <40 bits if you start making assumptions about device vendors. For example maybe the attacker has a VMWare exploit: enumerating that OUI (00:50:56) leaves them with just 24-bits of address space to scan, a measly 1GiB of traffic.

It's security through obscurity at best, i.e: not security. You shouldn't be relying on size of address space to protect you from anything. An IDS/IPS that alerts on abnormal ICMP behavior will be useful whether an attack is 1GiB of traffic in size or 1024EiB of traffic in size. (Also you don't even need automated scanning to find some juicy targets: I've seen a lot of routers on the edge of a prefix configured at ::1/64 and ::2/64 for instance.)