Q - VO2max, Lactacid Profile, Pulmonary Efficiency, Anaerobic Threshold, Hemoglobin Mass, Muscle Strength, Body Fat %: all these factors influence aerobic performances. Which is the decisive one for a competing cyclist? 

A - All those parameters are important, but none is decisive: having a VO2max = 80ml/kg/min, Hb = 16 g%, body fat = 4% is not enough to determine a champion; surely though, they are a good starting point. They are a necessary condition, but not sufficient. 
In any case, what is possible to measure through tests is only a small part of the several parameters, some still unknown, that make for high level performances. Every great athlete is too complex a "phenomenon" to explain thoroughly with today's scientific knowledge. 

Q - How much can Doping or the use of drugs improve aerobic performance? 

A - The majority of "doping" drugs does not actually improve performance; often many products impair it, especially in the mid-long term. For example, with regard to Gh, anabolic steroids and corticosteroids use, there is no serious scientific study proving an enhancement in aerobic sports. 
The fact that athletes using such substances report "sensations" does not mean that their performances have actually improved. 
Oxygen delivery enhancing drugs (EPO and similar) or methods (transfusions) improve performances by 2-5%, equal to about half of the increase in oxygen availability: in other words, if the Hemoglobin Mass (not to confuse with hemoglobin concentration!) increases by 10%, performance improves by about 5%. 
The fact is that equal or even superior enhancements have been constantly reported by the scientific literature, through the correct use of perfectly legal methods such as dietetic manipulations (CHO-loading, Fat-loading), pre effort Hyperhydration, altitude, Precooling, aerodynamic arrangements, superfluous weight reductions, pedaling gesture's efficiency improvements, or simply a better distribution of the effort. 
Athletes of all kinds, even top-level ones, not always pay attention to all these details, sometimes preferring the help of drugs. 
In all probability, athletes resorting to doping are less talented, less determined to face the necessary sacrifices, but also have a more limited access to information on how to improve legally. 

Q - Assuming that all top-level cyclists have knowledge of the best methods of training, nutrition, hydration, etc. and that all of them use world renowned coaches, why should they give up doping? 

A - The assumption that professional cyclists utilize the "best" schedules for training, nutrition etc. is definitely utopian and quite far from reality. 
There is ALWAYS room for improvement for the SINGLE ATHLETE, even if trained by the best coach (if such really exists...) or followed by the most expert nutritionist. 
The athlete must comprehend this truth and realize that resorting to doping is risky and illegal, and that he/she can achieve the same improvements through perfectly legal ways. 
Doping must also be fought from within the world of sports by proposing valid and credible alternatives to athletes.