Using private address is just a convenient way to set up easy invariant templates for FW rules. No more, no less.
If you add the fact that ISP used to not route RFC 1918, it used to work quite efficiently.
If you want the internet to continue to degrade into something closer to cable TV, then continue requiring central gatekeepers. If, instead, you care about the future of the internet, then please use globally routable addresses instead of the imprimatur we call NAT.
It has been used as a way to centralize traffic by some rogue ISP, and then because Large Scale Nating involve to hold in memory a lot of state considered bad practices because it was costing money to ISPs. (plus FW redondancy/HA in NAT require to synchronize states with CARP or CISCO techs).
But NATing behind the POP of the customer behind a public IP with the classical 3 ways filtering (corporate net, DMZ, internet) still enables templates to be easily shared and understood.
It is not NAT that sux. It is incompetent sysadmins the problem.
"Because it's easier" is a terrible reason to break the internet and limit the development of networking software such that proper direct-connections (in either peer-or-peer or client-server style) are useless and a centralized 3rd party is required to negotiate the connection and/or manage the NAT hole-punching.
You're talking about convenience for a specific set of tools, when the problem is about freedom to publish without middlemen.
> a way to centralize traffic by some rogue ISP
ISPs have little[1] to do with this. I'm not talking about centralization by the ISPs; I'm talking about how network software such as VOIP should be making direct connections once the address is known, which is impossible due to NAT. Instead, we have Skype with Microsoft in a de facto position of control over a lot of the "voice chat" ecosystem.
> enables templates to be easily shared and understood
I'm sure the file-format for those templates can be extended to support a placeholder/variable/macro for local addresses.
> NATing behind the POP of the customer
That's my entire point. This is how the internet was turned into a "two tier" system, where some hosts can use listen(2)/accept(2) usefully, but everyone else has to ask permission of the incumbent feudal lord for permission if they want to accept a connection.
You seem to prefer trading that ability for an internet that resembles the "cable tv" model instead of a network of equal peers (in the protocol). I hope having convenient firewall templates was worth it.
[1] other than dragging their feet on IPv6 for the last ~15 years, which removes the need for any type fo NAT