introducing new characters, instead of casting new actors in roles created by other actors, there is opportunity for new dynamics and story lines.

The downside of the Mary and Matthew relationship is that happy, married couples can only have so many plots between each other. There are other television shows where the couple in question finally did get together (e.g., Sam/Diane in Cheers, Rachel/Ross in Friends, etc.) and what was possible to write with the relationship became limited.