Boxing is a big part of MMA and of kickboxing, obviously, but when you open up the rules you introduce more techniques, and these have to be dealt with through cross training.
Boxing alone is not the best preparation for K-1 – Kickboxing training is.
Boxing alone is not the best preparation for MMA – MMA training is.
But boxing is as good a place to begin as any other for either rules set, and history shows that sometimes, just plain old boxing will get you over on some of the very best in these smaller fight sports, witness fat old Frans Botha bettering all-time greats Jerome LeBanner and Peter Aerts at their own game (sour grapes claims of injuries and ill-preparedness notwithstanding regarding the Aerts match).
Botha in his old age became higher ranked in K-1 than he ever was in Boxing.
Ditto for Butterbean Esch following his mauling of Japan’s Fujimoto.
Both of those bulbous bruisers were well past it by the time they crossed over.
Boxing is fundamental to either Kickboxing or MMA, and a good enough boxer is gifted with a level of boxing that is virtually non-existent in MMA or Kickboxing, self evidently, and does not have to master the other elements so much as he has to be aware of them, become rudimentarily proficient at them and learn to employ what he does exceptionally well to defend against and circumvent them, just as someone from any other background would have to do, and because boxers come form a much, much, much deeper talent pool of natural fighters, they will uniformly take the shortest amount of time to adapt.
If you set up old, used up, smaller guys like Arthur Williams and Vince Phillips to loose against bigger, younger guys like Iggy and Masato who represent the best you’ve got, that’s what you’re going to get; a showcase of your sport.
If you even the playing field a little, you’re going to see things like Mike Bernardo vs. Andy Hug happen, because boxing is the bigger country in this war, and while you can’t up your natural threshold of ability, you can very easily cross train technique, a reality that puts boxers at a huge advantage in crossover scenarios.
Of course, most every dollar paid out to fighters world wide is paid out through Boxing, and this would explain why so few decent ones ever give the smaller leagues a second thought.