People like to watch sports because they are a fair competition. If performance enhancing drugs are allowed, it would just be a competition determining who can cheat better. It would not be a test of sportmanship and physical abilities, but of who can pay for better drugs and more advanced treatments. No one wants to watch people using drugs and cheating.
Fans just want to watch sports. Fans don't really care about the long-term health of the athletes, or the ill effects of performance enhancing drugs on them. Fans want to watch their sports. Surely the majority of fans would have some sort of opinion on the matter, but to suggest that they would be impacted to the extent of changing their viewing habits seems silly. Additionally, you would be hard pressed to find corporate sponsors who would pass up prime advertising space over such issues. The allowance of performance enhancing drugs would have little if any effect on revenues, fan turnout, or corporate sponsorship.