The popularity of a mechanism does not necessarily correlate with the effectiveness of its security. The fact that we're using IPv4 with NAT+SPI versus IPv6 with SPI is simply an accident of history.
If IPv4 had been designed with 64-bit addresses, or even 48-bit ones (like Ethernet MACs, which we still haven't run out of), then NAT probably would not have been invented, and we'd be using 'simple' (SPI) firewalls (see Cheswick 1994).
> […] but I wouldn't trust it as much as the security of a non-routable network.
Non-routable networks are not more secure than routable ones with SPI; at best the two are equal IMHO. Non-routable networks may actually be worse because of a false sense of security: all it may take is one end-point compromise and the enemy is one the other side of the moat.