"Because I can log into my sshd, so can everyone else"...? We are not talking about weak crypto here (though AFAIK 5G crypto is designed to be decidedly not E2E), but rather access to the data processed by these systems. There is no particular reason why granting e.g. the NSA access to a stream of CDRs would immediately give others access.
I don't think anyone is saying immediately, but these are long-lived fairly static systems.
I'm going to assume that you probably update your server's SSHd at least semi-regularly and that if SSH turns out to be broken 20 years down the line, you will probably be switching to something better either manually or when you eventually replace your hardware and reinstall the OS.
This kind of infrastructure is meant to last for decades. Imagine if the original GSM contained a backdoor with the state-of-the-art crypto of the time. Would it still hold up today? Hell, you don't have to imagine - a mid-range smartphone these days can crack a lot of GSM traffic.
Besides that, there's also the problem that not only are these things usually not done with state-of-the-art tech, but leaks happen all the time and it only takes one mistake* for the privkeys to become known.
Why would the UK or Germany be interested in granting the NSA access? Those are some of the places where the US government is spending lots of effort to keep Huawei out.
So yes, any intervention like that seems to me to be less about "the Chinese can snoop" and more about "we can't".
If you're able to retrieve the private key, then yes, you're able to generate a working public key. If they even use a (pre-generated) private key. Often, passwords are even hard-coded in "firmware".
