It was always just sort of accepted, even by the Broccoli's, that James Bond in film is not one man. He is, in fact, each of the different men we've seen him as. Sean Connery played one incarnation, Roger Moore another, etc. Each new Bond was the successor to the last agent who was assigned the name to go along with it's correlating 00 designation. Skyfall crapped all over that and essentially invalidated all Bond films that came before Casino Royale. None of the pre-Craig films could have happened, meaning Casino Royale was a total reboot of the franchise. And that causes all sorts of problems on it's own, like M being the same person as in the Brosnan installments and such. It's a mess of Skyfall's making.
No Bond film had ever devolved into a Home Alone sequel until Skyfall. No Bond film had ever tried to pass off a Joker knock-off as the antagonist until Skyfall. No Bond film had ever shown Bond to be a lazy turncoat until Skyfall.
But that's the thing. Those are not always constants and Casino Royale showed that they probably shouldn't be. The fact that after Casino Royale they kept creeping back more and more to the old formula shows that none of the films since have been as inspired or intelligent. Casino Royale made a promise that the franchise couldn't keep. They rebooted the franchise in a really smart, timely way just to veer it right back to Brosnan/Moore territory.
The 'multiple agents' thing is just a fan theory, and there's evidence in the films that directly contradicts it. If you're inclined to view the Bond films as a continuity, then the opening sequence of
For Your Eyes Only is incomprehensible: Roger Moore leaves flowers at the grave of George Lazenby's wife, whereupon he is attacked by a figure that is apparently a really incompetent Blofeld in a wheelchair, going for some sort of revenge. Actually, pretty much everything to do with
On Her Majesty's Secret Service is incomprehensible, because George Lazenby's Bond appears to have the same relationship with Blofeld as did Connery's, and in
Diamonds Are Forever things continued with Connery as though they'd never changed.
Of course, if you're not inclined to view the Bond films as a continuity, then all those problems disappear. Creating some sort of unified lore explanation isn't the point. We're not talking about the DC universe here, we're talking about a bunch of British action movies whose creators borrow from each other and ignore each other as is convenient for them.
The same goes for the sorts of devices and ideas employed in the films. So Bond never did a 'Home Alone' sequence before? So what? There was never a major boat-chase scene in a Bond film until
Live and Let Die. Gadgets in general didn't take center stage until
Goldfinger, with Bond's briefcase making a minor cameo in
From Russia With Love. (Back in
Dr. No, all Q gave Bond was a standard-issue Walther PPK to replace Bond's beloved Beretta, and M was really the one who drove that conversation.) Knock-off antagonists? We've had plenty of those, too; anyone who believes that Franz Sanchez or Kananga were
sui generis is deluded, and Max Zorin was a fairly transparent excuse to get Christopher Walken to play a Christopher Walken character. And I don't even know what 'lazy turncoat' is supposed to mean, and neither do the vast majority of people who watched
Skyfall.
And about the last thing...uh, let me know when you find a Bond film with a main character that isn't all of those things that I mentioned.
Look, I agree with you that
Skyfall is overrated, that
Spectre was kind of bad, and that
A View To A Kill was hot garbage. I agree that there are a large number of terrible Bond films. (There are four categories of James Bond film: bad, hard to watch for me personally because of horrendous amounts of misogyny, both, and
Casino Royale.) But sometimes criticizing things goes way too far.
Skyfall can be overrated without being a betrayal of the franchise or whatever.