I don't think Senna has been choosen by nostalgia and I don't think Eric Boulier has got the time to take in consideration the colors of the car and Ayerton's memories.

Heidfeld has not given satisfaction and surely LRGP's fault, maybe they were expecting too much from him in comparison of Kubica's performances last year. He also had misfortunes this year with his car.
Senna is under contract with LRGP as a titular driver of the team. And yes, he brings money to the team.
Depending if Kubica will come back next year ( it is definitely out of question for the end of this season), Romain Grosjean had to finish his season in GP2 for a world whampion title (remember what happened in 2009, leaving a great season on GP2 for F1 with all the problems we know). So, who to replace Heidfeld at SPA and why now? Senna for money? Yes, it is possible but also the team needs to score points to reach Mercedes GP and to prepare a new driver for the next season if Kubica can't come back.
I am sure they would have prefer to replace Nick by Romain but he can't until Singapore. And Romain has been contacted by Toro Rosso. So Romain is important because he is a hope for the next years in Formula 1. And Eric Boulier wants a french driver in F1. But he is not his priority. He just want to score points for the team. Petrov has considerably improved his driving this year even if he is not at Kubica's level. But less faults, more constant in race, good competitor, motivated. The last disappointing races for LRGP was not because of drivers but underperformances from the team. I think that Petrov deserves his seat (I wouldn't have say this last year). Maybe LRGP take a risk by replacing Heidfeld at this stage of the season but Senna has also improved himself and he knows the car well with tests and friday practices (well, enough I think to qualify in Q2 and to make a race).
I am sure that if Senna doesn't give satisfaction to the team at SPA and Monza, he will be replaced by Romain Grosjean (who will make Friday practices at Singapore and for the rest of the season) before the end of the season....
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