For me, Massa joined a long list of drivers who had serious accidents and who were never the same again when they returned. I don't believe it's nerves or something like that, I think it's just that F1 moves on so quickly that any time away from it is going to cause you problems. When Massa had his accident he could have been considered the team number 1 driver and was getting the better of an increasingly unmotivated Kimi. When he returned he's missed out on a 6 to 8 months of driving and development work and suddenly finds himself having to match Alonso who, coming off the back of that year at Mclaren, had a massive point to prove.
I think Massa will still be able to find a seat in F1 and that Renault do look like they would be a good fit for him but for me, it wouldn't have been a big issue for Ferrari to ditch him at the end of the season.