That can be completely disputed....
1. You cannot guarantee that he got the virus from that ralley
2. It is possible that you got the virus from another source
3. If it was from the convention, Trump didn't force him to go to the convention
4. Trump didn't force Cain to not wear a mask and not social distance
5. Lets say Trump wasn't in office and didn't hold a convention that day. You cannot say that Cain wouldn't have contracted the virus the following week picking up McDonalds or some other weird way.
6. Lets say Trump wasn't in office, Cain could have been hit by a car last week and therefore, wouldn't be here today (plus a million other examples)
So your claim was completely disputed. You cannot say that he would be alive today if it wasn't for trump