Those you spot are not on a true collission course.
An aircraft on a true CC will appear completely stationary. In 600hrs I've never had one of those (evidently, as I am writing this) but have seen plenty which I felt a need to avoid. They might have missed by 50-100m, which looks very close.
The only foolproof solution, for those who think it's worth paying for, would be mandatory Mode C and TCAS.