I thought so as well. I went into every show after the merge thinking, "is this the week they get rid of Rob?" Yet every week he'd be safe, many weeks without even getting a vote against him.
The idol was a huge thing. During the show, we thought no one knew he at it. No one. I bet thousands of viewers forgot he had it - since the last time I recall the idol being discussed was when he tossed that last clue in the volcano.
It wasn't until the jury vote that we knew both girls knew he had it. They openly admitted that played a giant role in Rob staying at the final 5 vote. I assume they knew before that point which saved him from previous votes as well.
Someone in this thread mentioned Rob played Russell's game - just much better socially. It's so true that I believe it's an undeniable fact. Get a young female or two close to him. Make an ally with a strong guy. Find immunity idols and win challenges. Backstab when need be...proactively vote off threats.
The thing Rob did that Russell couldn't - is have people for the most part still like/respect him through the process.