You're not secure until you whitelist. And even that's not a guarantee; it's a necessary but sufficient condition. But systems which do not run signed, whitelisted code from boot time forward are as good as pwnt.
No, a whitelist isn't good enough. You can't anticipate an exhaustive list of the programs the user will want to run.
What you can do, however, is enforce a policy by which programs are required to provide machine-checkable evidence, also known as proof-carrying code [https://en.wikipedia.org/wiki/Proof-carrying_code], that they respect the system's safety policy.
