IMHO, interval blasts (at least in Cathe's workouts) serve a twofold purpose: to increase aerobic capacity by taking you to its limits; AND to build power in the leg muscles. And as your cardiovascular capacity is continually challenged AND as your leg muscles develop more power, what previously brought you to your limit subsequently brings you to . . . well, not your limit, because your limit has improved. Your anaerobic threshold has been raised, so to speak, and your legs can just plain do more. It's called the training effect; your body adapts to the demands placed on it.
Cathe said it best in her intro to Interval Max: "As you continue to do this workout you will get stronger and stronger, and be able to go longer and longer, not only in this workout but also in your other workouts, therefore burning more total calories." (I'm a little weirded out that I can paraphrase that closely.)
When I do blast-only stuff (and one of my fave Sunday routines is Interval Max warm-up, I-Max 3 blast only premix, Kick Max Boot Camp Challenge, and Boot Camp cardio only premix) I never have to decrease intensity. And in fact most times I extend out the blasts until the chapter flips to the next blast. It takes doing, but it's doable. And my resting heartrate is now at 45 bpm. Not bad for an old broad, and I have Cathe blasts to thank for it.
A-Jock