Hampus - no one said he was stellar, and no one said Renault sack people for fun. The point is that many people think he achieved a realistic expectation of getting as many points as his team-mate.
Renault selecting a rookie tells us that they don't expect to challenge Mercedes for 4th, and believe Petrov on his own can get enough points to stay ahead of Sauber. So that gives them the freedom to experiment with a rookie for the end of the season.
The rush of sponsorship announcements shows that cash was a big part in Senna getting the seat.
The strategic reasoning is that even if Senna scores no points, they'd be in the same position in the WCC, but have the added benefit of extra sponsorship.
They only needed to ensure they could pay off Heidfeld's contract for as little as possible. They messed that up and got dragged to the High Court. That shows how much cash was on offer from Senna.