The films have almost NOTHING in common with the TV show other than the theme song and name. They are radically different - as different as Star Wars and Star Trek. Both are sci-fi, but totally different concepts.
Exactly. Well put. The spy thriller genre has lots of sub-genres, and lots of different kinds of film. The TV series of M:I was one kind of thing. The first film of it was something completely different. While it's perhaps not as massive as the first film's wiping out of every character from the TV series but one (who turns out to be the villain), the opening of the second film, in which Hunt is hanging from a cliff somewhere and is given his latest mission through a pair of sunglasses, is almost as radical a - whatever word we want to call doing this if not reboot.

Yep--CR is a restart. The MI series has no restart--it only restarts from the TV show but once the movie series starts there has been no restart.
You must have watched a different film to me, then. For me, M:I was a dark, complex spy thriller that harked back to Cold War classics like SIX DAYS OF THE CONDOR. It was set in Eastern Europe, and featured a brilliant secret agent on the run from his own side, framed as a traitor. Like many Cold War classics, the head of the service turns out to be the traitor. It relied on its script, and twists in the plot, more than explosions or gadgets. It had lots of tradecraft, and a main character who was determined to prove his innocence and get to the bottom of some deep corruption within his organisation.
The second film could easily have been called something completely different, as could Hunt have been. None of the events of the first film are referenced, and he is now back working for the guys who tried to kill him without so much as a blink. Apart from the longer hair and slick clothes, he now has a fast car and a taste for beautiful women. He visits exotic locations. There are massive explosions and gunfights, with cues from Hong Kong films (unsurprisingly, as its director was a key proponent of that genre). The villain has a poison that will destroy the world. It's basically a standard Bond film with a bigger budget and John Woo directing. The tone, content and premise of the film are completely different to the first one. All that remains is an American agent called Ethan Hunt (who seems to have a different character and has forgotten what he went through in the first film), Ving Rhames, the title and the theme tune. That's it. CASINO ROYALE will have a hell of a lot more in common with DAD than that.