Large blocks of IPs are not part of the internet. 10. 172. Etc...
Plus the copy about not hammering other networks won't matter when you're trying to scan entire /64s that are behind one home router or something. That's gonna get noticed. You can't really scan IPv6 like you can scan IPv4, the math I gave is part of why. The code to do it is trivial, but the hardware just isn't there.
For example, someone with access to a backbone internet router could easily log src and destination ipv6 addresses, and sell the complete list sorted and compressed. Malware authors could then use the list to portscan for badly firewalled stuff.
There’s more than enough stuff on IPv4, IPv6 isn’t worth the effort.
FWIW shodan was setting up their own public ntp servers to track down v6 users.
First time to be called shady, though ;)
It is an accident of history that computing spent so much time at binary orders of magnitude that technology could catch up and make the previous limits seem small. At some point, the exponential curve takes off and no amount of physical resources can catch up to it, because our universe taps out at O(n^3). 2^128 is pretty much on the other side of that takeoff point. You can almost catch up to it... at the cost of the resources of an entire galaxy... or more.
(Technically it's even sooner than O(n^3) if you also have to avoid black holes, but since the relevant comparison here is "exponential" the details hardly matter here.)
Still, network address obfuscation really shouldn’t be a security control.