In one facet you are correct.
HOWEVER in this particular case, where it is proven that the ENSEMBLE surrounding the franchise is what the audience cares for then can certainly end that particular franchise.
Imagine Will Smith and Martin Lawrence announcing their going to do a trilogy of Bad Boys films, Martin dies suddenly during the filming of Bad Boys 4, you just can't replace Martin's character with Kevin Hart or Chris Rock and expect the franchise to run without missing a beat, why? Because the chemistry between the two leads of Smith and Lawrence are the EXACT reason WHY that franchise prints money. F&F prints money based upon the chemistry generated by Vin AND Paul, which in turn energizes the entire cast. The audience doesn't necessarily care about the races and explosions, they care about these CHARACTERS being put in the situations of racing and explosions. You lose Paul Walker and you've lost one half of the ESSENTIAL reason why the franchise works in the first place.
So you are correct in your assertion that the studios, executives, managers, etc primarily care about the financial dividends attached to the franchise, but in this case it's actually the CAST that provides those dividends.