Bond obviously got pretty comical 007 during the early '70s, and I can see why, as the franchise was a cash cow that was marketed for the masses, rather than the cineaste. The average Bond fan may have been quite a different audience than the one who'd go and see The Parallax View and Three Days of the Condor .
Brian: I've heard debates among Bond fans on this general topic. Some say why couldn't we have had a serious Bond in the early '70s? Afterall, there was Three Days of the Jackal (or name other serious spy movie here).
The thing is, The Day of the Jackal, The Parallax View, Three Days of the Condor, et. al. were all one-offs. (I ended up seeing all three movies when they were released.) They didn't have to worry about carrying on a series, etc., etc. With Bond, and I can only speak for myself, as an audience member I was looking for something else. By the time Casino Royale came out in 2006, Bond had been around so long, I *suspect* the general audience was more open to a more serious approach. But I also suspect that in the early '70s, had there been a Bond with deception, double crosses, etc., there would have been a bad raction. If we wanted dark, double crosses, etc., we could go see Three Days of the Condor, The Day of the Jackal, etc.
Also, as I noted in another reply in this topic, Live And Let Die was the first Bond film to have a worldwide gross that exceeded Thunderball (LALD's U.S. box office still trailed Thunderball's). There was no way once LALD came out, they were going to go dark for a while.