Let's just get it out of the way from the jump: we're not talking about the viability of the Fast and the Furious franchise. As of writing this, the latest installment, The Fate of the Furious is outpacing Star Wars: The Force Awakens at the worldwide box office. In terms of making money, Fast and the Furious is in better shape than ever.
What we ARE talking about, in this case, is whether or not the Fast and the Furious franchise has run out of gas both narratively and creatively. It may seem like an absurd notion to many of the hardcore F&F fans out there, but for the franchise's critics (of which there are equally many), these movies jumped the shark long before Dom and his crew were using their speed racing skills to take down nuclear submarines.
The question at hand is: Has the Fast and the Furious Franchise Gone On Too Long?
It's not a mystery what critics of the Fast and the Furious franchise think - they pretty much shout it from the rooftops every time one of these films is released. In the minds of the haters, Fast and the Furious jumped the shark a long time ago.
Even if a critic or hater liked the first film, the general consensus is that subsequent sequels have gotten so increasingly absurd with their action quotient that by Fate of the Furious, the franchise is offering something more akin to superhero movies than a action movie about gear-head street racers with a penchant for crime.
...And there's definitely some truth to those claims.
The first four Fast and the Furious films stayed pretty grounded in realistic (enough) street racing action and crime drama; however, by the time that Dom's crew was using Dodge Chargers to steal bank vaults in Fast Five, we had crossed into a whole new level of F&F experience. Since then, the stunts and set pieces have only gotten ore (elaborate? Ridiculous?), including an airport runway that never ends (F&F6), cars parachuting from a plane (F&F7) and cars being used to guide missiles into submarines(F&F8).
For those who are easily triggered about illogical or unrealistic action, these movies are (understandably) the new bane of their collective existence.
Fans of The Fast and the Furious franchise have an easy argument to make: All they have to say is "Look at the scoreboard."
The films have made progressively more money since being relaunched with Fast & Furious in 2009, with Furious 7 earning a billion and a half in worldwide box office returns. Some have argued that F&F7 had the unique boost of being the send off film for the late Paul Walker; however, with Fate of the Furious now outpacing even Star Wars: The Force Awakens in worldwide box office sales, it seems Walker's tragic departure won't stall the franchise in the least.
Some haters try to argue that big money does not determine a franchise's greatness (the 'Transformers Theorem'), however, those who support the franchise counter with the epic action (which consistently entertains, no matter how absurd), and in some ways, that's enough to get by.
Finally there's the diversity issue: part of why Fast and the Furious has struck such a big chord on a global level is because it was building a diverse cast of fun and interesting characters long before Marvel and DC and Star Wars all decided to follow suit. With that diversity comes an even wider fanbase - many of whom see F&F as one of the only (if not the only) major action franchises that caters to their respective cultures. The brand loyalty that creates is hard to break.