The case is starting now because key evidence was only revealed by Bernie Ecclestone this year. Before that, what happened was not suspected to have involved a crime on the part of the organisers, thus a court case would have been futile. However, there are crimes that sound rather close to what the FIA is suspected of having done.
In that case, "it's 15 years ago" may not be relevant (as far as I can tell, it isn't in Brazil and wouldn't be in France if the case was serious enough).
What the FIA states about when championships can be challenged might count against it, since at this point it's the FIA's own actions that are alleged to be the crime (otherwise, anyone could commit a crime and justify it by hiding their crime until after the timeout). The brevity of allowance the FIA has (2 weeks) is much shorter than permitted in most walks of life, especially in cases where criminal action is suspected. There's also the part where, if the FIA committed wrongdoing as part of Singapore 2008, the FIA awarding the Piquets immunity becomes another point of attack against the FIA. (since they effectively became co-conspirators to hiding the FIA's behaviour by taking all the flak).
The "maybes" are not relevant unless the alleged crimes involving the FIA had knock-on effects that demonstrably assisted Massa (and would also have to counter the point that had Ferrari chosen when to pit rather than had it forced-rushed by the Safety Car procedure, Massa probably wouldn't have escorted a fuel line down the pit lane).
I think at least 3 championships in the 21st century* become open to challenge on a similar basis if this goes through. The older the championship is, the safer it is, simply because there are fewer witnesses in a position to take sides (due to Charlie Whiting and Max Mosley's deaths, even 2008 is tricky, let alone anything preceding that date). 1994 is rock-solid safe because Damon Hill has consistently said that the title should in reality have been Ayrton Senna's, and with the best will in the world, no court in the world has resurrection as an available remedy (which would be a pre-requisite for Ayrton to get the necessary points even if every title contender consented to a re-run for old times' sake).
* - 2012 is at risk because Bahrain, by FIA regulations, should not have been held for reasons of safety and political advertising by the promoter (each regulation requires the removal of the race in their own right as the consequence). Both infractions were formally pointed out to the FIA at least a week before the weekend began but the FIA chose to ignore them because "it's too late now" (patently false). That loses Vettel his first win of the season, but only loses Alonso 6 points. This means Alonso (272 points) takes his 3rd title, defeating Vettel (266 points). That one's the easiest to explain and probably the easiest to swallow, since at no point is any driver or team's conduct challenged, only the FIA's (and the promoter's).
2022 is at risk because Red Bull broke the budget cap in 2021 and was fined less than the amount subsequent filings indicate Red Bull overspent (let alone any penalty that is supposed to occur). If you think removing a title after the fact is difficult, try getting money unspent. Since this would likely remove Red Bull entirely from the standings, that would make Leclerc champion... ...except that Saudi Arabia 2022 happened on a track not meeting FIA regulations for a circuit of any grade and thus all results from it would need to be removed, and Japan 2022 is separately challenged for multiple breaches of the Bianchi settlement, that put drivers at risk. Depending on whether that is resolved by doing nothing, chopping off the start of the race, the end, both or simply deleting the event from the records, affects whether Leclerc or Russell ends up champion.
2021 is the most complicated one because of the lawyers involved. The races I know to be in dispute include Belgium (according to the FIA regulations, the race should never have started), multiple races with misuse of flags concerning cranes and crews, Saudi Arabia (see previous paragraph) and Abu Dhabi (the dangerous Safety Car wave-by that was against FIA regulations). That's aside from investigation into possible breaches of neutrality regarding Red Bull and the previously-mentioned overspend problem (debate about the contribution of mainline car from 2020 spending vs upgrades from 2021 spending mean it's not open-and-shut as to whether that's the slam-dunk 2022 is, hence the relevance of the neutrality part). I haven't even tried computing how many candidates for champion there are in this case, although Hamilton obviously is the most likely if any change occurs.