We talk about drivers like Heidfeld, Kubica and Massa and their glory days (notable accomplishments) are from almost a decade ago. If you focus on just Kubica and Massa, they both had brushes with greatness (including Massa almost being WDC) and then accidents that put their careers into question.
Massa was able to recover (however I believe it did impact his ability for awhile) and for Kubica the injury was generally considered career ending for quite a long time. While Massa is only a handful of years older than Kubica at this point we are expecting Massa's retirement as being imminent (especially given he actually did retire!). So if Kubia was driving today people would be wondering the same thing. When will Kubica retire?
Who would be older than Kubica (assuming Massa retires to free up the seat at Williams)...
* Kimi at 37 who is WDC and even at that some here clammer for his retirement.
* Fernando at 36 who is multiple WDC and again, many wonder if he is ready to quit.
* Lewis at 32 (roughly same age) who is multiple WDC
So if Kubica was still driving today, I suspect that unless he was a previous WDC, his F1 career would be in it's twilight as well. Could anyone really restart an F1 career at this point purely based upon the potential that was shown many years ago? If had been successful in other top level racing during that gap then maybe, but while I don't follow WRC, he seems to have been just "OK" recently (granted, luck can be a big factor in Rally). But he is just a big question mark IMHO.
I would love to see the Cinderella story for a Kubica return, but it would have to be a big risks for anyone to really consider it.
Richard