I have quite a bit of
martial arts training under my belt and often do kickboxing with people as part of their workout. It is a great cardio workout. The cardio kickboxing classes are a good workout, but I prefer actually having people hit pads and everyone I train has said they prefer it to (maybe they just like hitting me though lol). But yeah, kickboxing counts as cardio. Anything that gets your heart rate up and keeps it up does. It does not replace weights though. There really isn't anything that could replace weights (
resistance training in general I mean) with any kind of effectiveness except maybe something like gymnastics.