Hmm, its a decent thought. I suppose Tobi simply got too confident over time. He saw task after task being completed and simply held hope (if Tobi can have hope lol) that his tasks would be completed as planned. I think that up to Nagato's death, he completely expected things to work out. After Nagato's death, I think he was tired of letting others do the work for him and decided to act on his own.
The bottom line, i think, is that Akatsuki was comprised of the best shinobi he could find in the world. He could have hired a second string, but they would likely have not been half as powerful as the first string. That being said, I guess you could say it was plot no jutsu, but I think the best explanation is that Tobi got impatient and since his first string got him really close, he thought he'd take it the rest of the way.
