Not commenting on the whole IPv6 vs IPv4 debate but simply answering your question...
There aren't enough IPv4 addresses, so any ISP using IPv4 addresses is going to give 99.999% of their customers exactly one IPv4 address. Not ten. Not two. One.
So NAT has to work. Grandma has nothing to configure because either NAT works or grandma is calling her ISP to ask why her tablet ain't working.
So when the customer gets exactly one IPv4 address, the ISP is forced to hand a router doing IPv4 NAT. They have no way around it.
While if you take an ISP handing out hundreds of billions of IPv6 addresses to each customer, well... They are not forced to hand a router which does proper firewalling.
It's not a question of whether it'd be easier for the ISP to give a correctly configured IPv6 router firewall vs handing an IPv4 correctly doing NAT.
It's that when they hand one IPv4 address, they don't have the choice. NAT must work and there's no way around it.
I'm sorry, I don't follow. Our grandparents have working, secure NAT setups that no human being needed to configure. I've never once seen a "firewall ruleset" configured by a non-geek.
Really? I changed ISPs the other day. Got my new modem/router. You know what I did in terms of firewalls? Nothing. You know what I have? A firewall that blocks all unknown incoming connections. Does it work? Yes. Did I even have to do anything besides plug in the box? No.
heck, they even inventend protocols to do automatic NAT setup (UPNP) because configuring NAT by hand confuses people a lot.