I love story progression. It's part of the reason why Sims 3 has so much appeal to me. I want to be able to see my friends' and neighbors' lives change--watch them age, die, have children, get new jobs, lose jobs, etc. But, I do not like that families moving away is inherent in story progression for this very reason. Once they move, they are gone and so I don't get to see them age, change, and die.
I propose that in a patch having families move is made an on/off option in story progression OR if they do move for the chances of a move to be minute (1/1000?). At the moment, I'm seeing my custom families move within days after I place them in my neighborhood. I think both of these options are perfectly reasonable and would make those unhappy with moving sims content without messing with the gameplay that other simmers prefer.
Thanks for your time!!