Here is something that the 5live boys considered - I think it was on the chequered flag podcast (which by the way you should check out - it's excellent).
Let's say Heidfeld and Petrov put in some strong performances until, say, race 15 or even a little later. Let's imagine that in Petrov's case, his mistake in Malaysia was not a true display of how far he has developed mentally. If Heidfeld and Petrov keep turning in top 6 performances for the rest of the year, where exactly does Kubica stand in all this? It would be an entirely different scenario to that of Massa, since neither of his replacements amounted to anything, and so they stuck with him, partly, yes, out of loyalty I imagine, but partly also because there was probably no-one else they would really consider.
Petrov has a 2 year deal if I remember correctly. Of course, he brings sponsorship. Under this case, where he is a consistent points scorer in 2011, you'd have to assume that they'd keep him for those 2 years.
Heidfeld though is different. If he gets some podiums, even a win maybe, then would they really replace him with Kubica for the final few races, if Kubica is fit to return? Kubica's return seems unlikely this year from what I've read. There is no doubting Kubica's ability - in fact I would have him as one of my top 5 or 6 drivers in the sport right now. But would he really have done much more than Heidfeld at Sepang? I really doubt it.
Then if we consider that Massa almost certainly has had somewhat of a drop-off since his accident, confused slightly by the domination from his teammate, we might also assume Kubica could face a similar period of time where he struggles to return to full speed. Possibly he is mentally tougher than Massa, but you can never tell for sure. Perhaps even there would be a slight physical handicap in his hand which loses him that last tenth of a second.
Let's say Renault are content with the driver pairing they have now, at the end of the season, would they really take a risk on bringing Kubica out, purely because of his previous assumed abilities? I'm not entirely convinced they would. People will say about this Renault car, "what if Kubica was there, what could he have done", well I'm not really sure he could've done much more than the current two, if anything.
Formula 1 has obviously changed so much in the brief period of this season that Kubica may fall out of sync, he'd lose the knowledge of the tyres, the car, etc etc. It's a very different proposition to come in at, say, Korea, than testing for a few days before his accident.
So where then would that leave Kubica? I'd assume a team somewhere would snap him up hoping he'd lost none of his drive and ability. But whether Kubica will ever be a true front runner again, I'm not sure. I have my doubts, and that is quite a sad proposition.