Senna originally planned to move to Williams for the 1993 season. Williams driver Mansell retired from Formula One after his world championship title in 1992.
But Senna was surprised by a former rival: Alain Prost, who re-entered Formula One with Williams after a year out, had made arrangements. Prost’s contract stipulated that Ayrton Senna could not become his team-mate. This meant that Senna’s Williams option for 1993 was off. He had no choice but to stay at McLaren if he did not want to miss a year. 

“But I refuse to walk away from a fight. That is my nature. I will fight to the end and whatever happens, whatever the cost so that for once we can bring justice to our sport.“ Ayrton Senna said this in the wake of the press scandal that erupted after the incident at the 1989 Suzuka Grand Prix

The 1989 Formula One World Championship was dominated by changes: From now on, turbochargers were no longer allowed – Honda was forced to rely on naturally aspirated engines. In the previous season, naturally aspirated engines were still disadvantaged on the racetrack: Those who drove naturally aspirated engines usually trailed the turbochargers.
Honda, however, developed a naturally aspirated engine that was in no way inferior to a turbocharger – except in driving comfort. The driving experience of the turbocharged McLaren MP 4/4 was gone.

Senna’s dream of becoming world champion with the Lotus team was over in 1987: Although he was now driving on Honda engines, which gave him an advantage, the environment at Lotus no longer suited Ayrton Senna.
Senna already had plans, but his team had anticipated Senna’s steps: Brazilian driver Nelson Piquet was signed by Lotus for the 1988 season – Senna was not informed in advance. Since Senna did not have a deal with another team at that time, his future was uncertain…