Butler wouldn't be involved in a hypothetical sign-and-trade for Giannis, let alone any star player. Doing so would tarnish Miami's reputation as a franchise, considering Jimmy chose to sign with them in free agency. It will have to involve Herro, Nunn, Robinson, Bam, etc, if it does go the S&T route.
A lot can happen over the next 365 days. There was a period of time in early 2009 when nobody could even imagine the possibility of Lebron leaving the Cavaliers for another team. If you don't recall, that was the season the Cavs won 66 games and won dominantly in the first round. How could a home-grown hero walk away from that?
Yet, we know of the disaster that took place in Cleveland only a mere 14 months later.. despite the fact that Cleveland once again won over 60 games the following season and looked as dominant as ever.
If the performance of the Bucks is of that which is 'underwhelming' this coming postseason and next postseason, and the Bucks fail to make the finals both times, I certainly wouldn't discount the possibility of a disgruntled Giannis getting pryed away from the team he once loved dearly.
You cannot tell me another exit at the hands of the Raptors, or even the Celtics or Miami for that matter, over the next 2 calendar playoffs doesn't spark the probability of Giannis bouncing.