While many have lots of concerns about the relevancy of this series, it continues to be one of the few television series still in production which does not feel it necessary to fill the hour with murder, mayhem, violence, sex, and ridicule. This makes it a safe choice for many families who want a bit of entertainment together as a family. It is the only choice for the family on UPN, as far as I am aware.
For a period of time, 7th Heaven was doing 'topic of the week' episodes in the after school special vein. Some people like that style; others do not. The current season has focused more on the lives of the various family members, which is something that fans of the series wanted. However, as time has gone on, at least three of the actors playing the family's children have moved on to college and film work. This leaves many of the fan based frustrated. There's basically 2 of the original 'children' left in the series, and one of them is no longer a child. The addition of the twin boys 4 years ago as babies will probably pay off next season (if another season is approved) as they will be 5, about the age of the youngest child during the first year of the series. More antics of kindergarteners - multiplied by two - are expected.
Can 7th Heaven survive another season? I expect, due to the limited competition in this demographic, that it can. But SHOULD it survive, or should it bow out as gracefully as possible? That remains to be seen.