If you increase exercise intensity, the body will adapt to it (given sufficient rest). That's why many programs recommend an escalating level of intensity over time, so that the body is continually challenged. However, there is a ceiling to this level of adaptation for both aerobic exercise and resistance exercise: a person can only achieve so much aerobic fitness and can only lift so much weight.
Knowing that there is a ceiling to eventual progress puts exercise intensity in a different light. If ever-increasing intensity is necesary, how do athletes who have hit the intensity ceiling (their maximum performance) continue to stay in shape? It seems more likely to me that exercise must pass a certain intensity threshold to be effective, and it's not the case that exercise intensity must constantly be increased.