They are good-guy superheroes fighting international villains now. Take it or leave it. "Grounding" the series is incredibly unlikely since the box office numbers started to skyrocket with Fast Five's bank heist on wheels. Making the family into actual good guys after that has done wonders. I personally don't find it any more ridiculous than the goofy portrayal of criminal L.A. tuner-car gang subculture in the original or the straight out of a rap video vibe of the second one. I think the first 3 or 4 had much more limited audience potential because they were the teenage boy wannabe car-guy fantasies (also see the popularity of games like NFS:Underground). Now they are much more accessible action-comedy movies with a big lovable cast.
I'm confident that the majority of people watched the latest one because Vin is a badass superhero, The Rock is maybe the most charming/likable action star on the planet right now, and Jason Statham is always cool (and bonus for having done his own crazy car movies prior). Oh, and hey, there are still some cool cars, awesome stunts, and great comedy derived from the chemistry of the cast.
I've seen all the movies and Paul's death has absolutely zero bearing on whether or not I'll see future sequels.
