We've got a brick right outside the Magic Kingdom entrance. We bought it in 1996 and when the security tables went up, I thought we'd lose ours. But it's still there. However, Disney does have the right to take up any of the bricks if they need to. I don't think they would be a massive bulldozing just for the heck of it, though. But if a brick was in a site that was under construction, then it would be removed and not replaced.
Funny thing - when we last visited our brick, my husband said it looked walked on! Wasn't that the purpose?