Six missions are more than enough to finish the plot. Even if there’s one mission for each of the Partners, the Constant and possibly Grey there’s still one mission left.
We’re probably going to kill the Partners and other important Providence members in the first missions and then chase after the Constant until Grey’s sudden but inevitable betrayal setting up the very final mission.
And as far as ridiculousness goes: The first game had a mad scientist creating his own clone army, the second game had a Russian arms dealer who wanted to steal and sell an atomic bomb and Blood Money had a rival assassination agency with Albino clones who wanted to kill the US President as part of a plane to ensure that no-one else has access to cloning technology.
A cabal of rich money makers trying to manipulate events in their favor so that they stay/become more powerful and rich isn’t that far-fetched all things considered.