> We're never going to run out of legacy unpatched devices, and today's shiny new devices are going to be tomorrow's tech debt.

It's not about new or old. The devices that don't have any reason to be globally reachable never request it. New ones would do likewise.

Devices that are expected to connect to random peers on the internet are going to do that anyway. It's not any more secure for them to do it via hole punching rather than port mapping; causing it to be nominally outgoing doesn't change that it's a connection to a peer on the internet initiated by request of the remote device.

> There's no way in hell enterprise admins are going to give that kind of control to random devices, and home users aren't going to have the skills to cull vulnerable devices from their networks. So who's going to use it?

Enterprise admins can easily use it because they have control over the gateway and how it answers requests, so they can enable it for approved applications (in high security networks) or all but blocked applications (in normal networks), and log and investigate unexpected requests. They should strongly prefer this to the alternative where possibly-vulnerable applications make random outgoing HTTPS connections that can't easily be differentiated from one another.

Whether they will or not is a different question (there are a lot of cargo cult admins), but if they don't they can expect their security posture to get worse instead of better as the apps that make outgoing HTTPS connections to avoid rigid firewalls become the vulnerable legacy apps that need to be restricted.

Home users are already using NAT-PMP or UPnP and this has only advantages over those older solutions.

> instead of creating a mapping allowing essentially unrestricted access from everyone, have the app use an out-of-bounds protocol to allow a single incoming connection from exactly one vetted source. You get all of the benefits of a direct peer-to-peer connection with none of the risks of exposing yourself to the internet.

There are significant problems with this.

The first is that it's a privacy fail. The central server is at a minimum in a position to capture all the metadata that shows who everyone is communicating with. It's made much worse if the payload data being relayed isn't end-to-end encrypted as it ought to be.

But if it is E2EE, or the server is only facilitating NAT traversal, the server isn't really vetting anything. The attacker sends a connection request or encrypted payload to the target through the relay and the target is compromised. Still all the risks of exposing yourself to the internet, only now harder to spot because it looks like an outgoing connection to a random server.

Worse, the server becomes an additional attack vector. The server gets compromised, or the company goes out of business and the expired domain gets registered by an attacker, and then their vulnerable legacy products are presenting themselves for compromise by making outgoing connections directly to the attacker.

Doing it that way also requires you to have a central server, and therefore a funding source. This is an impediment for open source apps and community projects and encourages apps to become for-profit services instead. The central server then puts the entity maintaining it in control of the network effect and becomes a choke point for enshitification.

Meanwhile the NAT traversal methods are ugly hacks with significant trade offs. To keep a NAT mapping active requires keep-alive packets. For UDP the timeout on many gateways is as short as 30 seconds, which prevents radio sleep and eats battery life on mobile devices. Requiring a negotiated connection is a significant latency hit. For real peer to peer applications where nodes are communicating with large numbers of other nodes, keeping that many connections active can exceed the maximum number of NAT table entries on cheap routers, whereas an open port requires no state table entries.

> What's left is a handful of nerds running servers for obscure hobby projects at home, but they are more than capable of setting up a static firewall rule.

The point is to allow interesting projects to benefit more than a handful of nerds and allow them to become popular without expecting ordinary users to manually configure a firewall.