Another would be "Do one thing and do it well", which is Unix philosophy.
What we do need is a proper replacement for roughly the things OpenVPN plus PAM can do. A VPN plus some user and key management.
I'm not arguing that Wireguard has an obligation to tackle that problem themselves. I'm arguing against your assertion that VPN access should be completely decoupled from ensuring endpoint security.
All they're good for is checking that unpatched (but not yet exploited/evil) endpoints can't connect to the network, which is of marginal benefit compared to allowing them to connect but requiring they patch before accessing risky resources (like the internet or email).
Anyway. Like I said, I think Wireguard is amazing - I used PiVPN (which can be installed on any .deb distro) to set up a simple gateway for my laptop and phone to be always connected for DNS and local-network access. I'm very grateful for its architecture and simplicity in that regard.
I do agree with your overall sentiment, though. The next step is to make a higher level protocol on top.
You are going to want to try to avoid that problem, while making your tool still useful. Wireguard hits the sweet spot particularly well.
"Oh, I'm afraid the EasySecureAuth wireguard server doesn't support the AndroidWireClient client when using a Yubikey version 1. Either use version 2 or switch to iOS."