A remote code execution vulnerability in GNOME

> On the other hand: OpenSSH also uses seccomp, and is implemented in C, and handles untrusted user input in a privileged environment. So, apparently for some true Scotsmen, like the OpenSSH developers, it seems to be possible to not fail to implement secure software in the C programming language. Though, that might be the exception which proves the rule?

This is a poor example, because SSH has a functional requirement to offer remote access to an authenticated/authorized user. If you sandbox it to death, then it can't do its job, so you have no choice but to take a more permissive approach. On the other hand, the vast majority of software has no such functional requirement, and therefore can be sandboxed much more aggressively than SSH servers.

Remember: There's really no such thing as absolute security. There's "harder to hack" and "easier to hack." Sandboxing is, ideally, a defense-in-depth measure. Software should be designed to avoid failing or misbehaving on malicious inputs, regardless of whether a sandbox is present, but no software is perfect, so we sandbox things to further mitigate the risk.