Werdum is a great hw. If he beats Cain, he has beat 3 OF the greatest hw's ever, not THE 3 goat hw's.
Beating Fedor was amazing. Losing to Overeem in a safe way was smart. Beating Nelson was decisively crucial to his level of UFC fan interest.
When Big Nog lost, he was supposed to lose. Defeating the prospect Browne was a huge victory. If Werdum beats Cain, he needs to defend the belt more than any hw has ever defended it, to be considered the g HW oat.
Personally, I would consider him the goat if he beat Cain, avenged his loss by FINISHING JDS, and then held a highly active, dominant hw reign. Then he'd be goat b.c. let's be honest, MMA was more brutal before, but it's harder now (and still pretty damn brutal).
P.S.- UFC needs openweight and superfights, but they keep peddling TUF for exposure in marketable countries...