Why would they move?
I'm sorry, I grew up with this movie and love it as much as the next 80's kid does, but if your son went missing and has been gone for 8 years, why the heck would you move out of the house where he lived? I know they tell David that they had him declared dead, and I've never had a child go missing so I can't speak from experience or anything like that, but even if you've given up hope, wouldn't a part of you want to at least stay in the house where he grew up, if not for holding out hope, but also because that's where your memories of him are? I can understand not wanting to be reminded of your lost child with every turn in the old house, it just seems like bad writing to me - and just an excuse to have some "drama" or "action" for when David comes back to his parents house and there are 2 strangers living there...
maybe it's just me...
We are all graveyards, we squat amongst the tombs of the people we once were