My question is: How much can one deviate from the topic sentence before transitioning to a second paragraph becomes a better idea?
For example, if a topic sentence consists of an assertion, then the reader will assume that the supporting sentences list reasons that would convince them of that assertion. The paragraph will be coherent if I then proceed to list positive reasons for why the assertion is true, but I was wondering if it is possible to insert something like a comment to emphasize the strength of one of the reasons of a supporting sentence, such as "after all, if this were not true it would be...".
Such a comment somewhat deviates from the topic while still ultimately supporting it since it provides a negative reason for the contrary rather than a positive reason for the assertion. In that case, would a negative reason disrupt the flow of the paragraph, and would it be a better idea to transition to a separate paragraph in which the focus is on negative reasons for why the assertion of the previous paragraph should be accepted as true?
Perhaps I'm using a bad example, but this is the best I could come up with.