The original work was by Huggins and Hodges in 1941 which showed that Testosterone drove prostate cancer. Given that castration stopped the cancer (for a while anyway), it was assumed that the higher the level of Testosterone, the more likely and faster the cancer would grow. However, there was no research showing this.
Abraham Morgentaler and others looked at this again in the last 10 years, and instead formed the saturation theory. That is that once your Testosterone reaches the lower normal limit, extra Testosterone doesn't increase the risk of prostate cancer. Thus the lower normal Testosterone level is a saturation level, above which no extra risk or growth of the cancer occurs. It's still recognised that castration levels of Testosterone prevent the growth of prostate cancer, which is the principle of hormone therapy treatment. However, some data suggests that low levels of Testosterone but not as low as castrate levels might actually increase the risk of prostate cancer.
GIven that low Testosterone might be a risk factor for prostate cancer, and low Testosterone is certainly a significant risk factor for cardiovascular disease and diabetes/metabolic disease, it's probably not something men should strive to achieve.
After many years of refusing men any access to Testosterone Replacement Therapy (TRT) who have low Testosterone after successful curative treatment for prostate cancer out of fear of recurrence, research showed that TRT doesn't increase the risk of recurrence. This also fits with Morgentaler's suggestions.
So it's not obvious to me that Testosterone needs any moderation, other than for men on hormone therapy.