"Years later, after leaving the presidency of FIA, Balestre admitted to having acted to benefit Prost in 1989."[1]
https://en.wikipedia.org/wiki/Jean-Marie_Balestre
Having said that, Prost was the better racer, and Senna benefits from the halo effect of having died in his prime. He's got some great quotes, and he was definitely mistreated at times which increased his standing as a folk hero, but he was outclassed at almost every step by Prost.