Oh the legend of the '84 Monaco GP…
Senna had not lead the race at all. Prost had led most of it until he had engine troubles and was passed by Mansell, who crashed soon after.
It is questionable that Senna would have won the race. He had suspension damage that would have caused him to retire soon after the race was stopped. He was also being caught up by Stefan Bellof, who was faster than both Prost and Senna.
Prost actually was harmed by the early race stoppage. He only scored 4.5 points, instead of the then-usual 9, for his win because the race was called early. Had he finished 2nd of a full race, he would have scored 6 points, and would have won the championship (which he lost to Lauda by .5 point)
As for your conspiracy theory, let me just say 1. it was the Monaco GP, not the French GP, 2. Blame Porsche if you want, since the race director was Jacky Ickx (a Belgian, not a Frenchman) who some people say called the race too early because he raced for Porsche (the famous Rothmans one). Porsche was the engine supplier for McLaren.