>f your reason for deploying a VPN is because you fear to expose unknown bugs in sshd to the Internet the same could be said about every vpn solution.
Yeah. You might have a VPN zero day - but then you still have to get into the other SSH servers. Two zero days simultaneously active for openssh and your VPN solution? Pretty unlikely, especially public ones. Someone burning two private zero days on you means you're an incredibly high value target and neither of these would suffice as your sole defense to begin with.
The rest of your argument, if I'm understanding it correctly, is that you think people will get more lax with securing SSH on a box only reachable via VPN than if it was reachable by port knocking? It's possible, but I don't know that the evidence really shows that - lots of comments on this article are along the lines of "i set up port knocking and I've never even seen a malicious ssh connection attempt since then!" - no details of the rest of the security measures they've got in place.
And yeah, going from 'I set up a VPN to connect to my web servers via ssh' to 'I have VPN access to a whole network with all sorts of things running on it' is a big step up, but I don't think it's really in the boundaries of this discussion. Port knocking was never going to be a replacement for a larger VPN deployment, and when you're opening up network access to a wider range of things then how you approach things definitely needs to change.