It's still a serious issue even with a sandbox, both because of the endless stream of kernel vulnerabilities and similar issues in userspace processes the sandboxed processes communicate with to get work done.

Chromium's use of seccomp-bpf is solely to crack down on kernel vulnerabilities, as it's an additional layer over a sandbox already providing all of the security boundaries they need. It moves things along pretty far, but there are still at least 1-2 holes found every year.

It's definitely an improvement over browsers like Firefox where there are at least 3-4 unmitigated remote code execution vulnerabilities fixed every six week cycle...