Even if Itachi had no illness, he would have eventually become blind. This means Itachi would not be able to keep up with Minato's speed. Itachi wouldn't gain the EMS even if he were to stay alive since Sasuke would not have gained the MS if it wasn't for Itachi's death nor would he want to steal his brother's eyes.
On the other hand, Minato still had a lot of potential in developing ninjutsu as said by Jiraiya. Even the Raikage claimed that Minato was an unsurpassable shinobi. It was hinted that Minato was going to develop the rasengan even further by adding his own chakra nature to it which would no doubt make it even stronger. Being Jiraiya's previous student, there were also good chances that he was working on mastering sage mode. Doing this would allow him to develop an even more powerful ninjutsu arsenal along with even greater reflexes and sensory abilities. This would make it even easier to dodge Itachi's attacks and counter genjutsu. Especially considering Itachi would be blind, Minato would be on a whole other level.