Sure, but the failure modes of NAT is pretty good and the NAT is hard to misconfigure.
That the crappiest firewall could do the same is missing the point. People back in the day didn't have crappy firewalls, they had nothing.
NAT was and is a brilliant first step that everyone has nowadays.
UPnP tries its best to make it worse though but the existence of UPnP is alone proof that a crappy or good firewall wouldn't have been better than NAT for the common user.
That the crappiest firewall could do the same is missing the point. People back in the day didn't have crappy firewalls, they had nothing.
NAT was and is a brilliant first step that everyone has nowadays.
UPnP tries its best to make it worse though but the existence of UPnP is alone proof that a crappy or good firewall wouldn't have been better than NAT for the common user.