The FIA has the power to revoke titles. This has happened countless times in other major sporting events years after they took place, like the Olympics or Tour de France to name a few. The complicating factor is that Lewis and McLaren didn't cheat, at least not in 2008 that we know of (I'm look at you 2007). Since Renault did, that would in many other sports lead to Alonso's win in Singapore being retroactively struck and the result being nullified. But that would make Massa the WDC. I know that over the course of a season, no single race makes of breaks a champion, but this one race really could make a difference and there is precedence for this kind of thing in other sports. I'm not saying that this necessarily should happen though.