I have yet to see a router that allows that forwarding unless explicitly configured. Still, i'm using mostly openwrt/opnsense/mikrotik
Default is to disallow/block forwarding packets from public wan to private range lan.
ISP can still inject packets on ports that NAT opens if it spoofs the source address/port, so you still have some validity to argument.
It's true that almost everything comes with a firewall rule that blocks new connections from the WAN to the LAN, so in practice these connections will be blocked on most things by default. But they come with this rule precisely because NAT doesn't do the job.
Cool, me too :)
Anyway, the other side of the argument:
It is the default and default is secure. Users don't have to reason about it, they can assume it works, how doesn't matter and they may lack training/willingness to figure out.
You can't say the same for IPv6 where default is allow (have things changed?, havent checked in a long time)
NAT contributes nothing to your security in this scenario, and instead makes it harder (not easier) to understand and reason about what your router is doing.
That's the thing, it's not the default, default is public ipv6 for everyone and its the users duty to configure firewall...
I could definitely set this up easily, someone like my parents or friends would ask me 'what's IPv6?'
That's precisely why routers come configured with a firewall that blocks inbound connections from the WAN -- because the protocol itself doesn't have a firewall by default, and neither does NAT.