If any program can potentially be malicious (which is the effectively the case today with any downloaded software), then sandboxing is exactly the pertinent action - provided that the sandbox is tight enough.

I should have elaborated. If a program is known to be malicious, or should be treated as malicious, then it should probably be terminated. Given a potentially malicious program and no easy way to determine (lack of) malice, sandboxing is a reasonable measure.