For someone to be a danger to Michael in terms of what they could've told law enforcement authorities, for example, I don't think some one who knows a handful of intimate details about Michael's activities is any les dangerous than someone who know almost everything, as Tom did.

I mean, it wasn't as if Tom was gonna go off and start his own family and become a rival of Michael's.

The only potential danger Tom presented was in what he "knew".

Suppose Tom had been arrested? Would Michael have had him killed then, because he feared that Tom would cooperate with law enforcement?

And take Kay, for example: While she obviously didn't know nearly as much as Tom did, she certainly had to have known plenty that could've helped law enforcement, and Michael didn't have her killed.

The roles of Stracchi and Cuneo in the Five Families War against the Corleones was never made perfectly clear, but assuming that they were on the enemy side as well, who did Michael ever kill because they presented merely a potential danger?