The problem is that you guys are assuming the existence of a single timeline, where any actions in the past destroy/overwrite the original timeline. That is the Back To The Future version of time travel that is very popular, but there is another theory of time travel which posits that changes made in the past only prevent the time traveler from advancing towards one possible future. The original timeline still exists and runs parallel to the alternate timeline. This is the rule of time travel established in The Terminator universe. When John, Sarah, Miles Dyson, and the Terminator destroy the Cyberdine building and change the date of Judgement Day, the Terminator doesn't disappear out of existence because the future was changed. That future that he came from still exists in an alternative timeline, and the Terminator will still have his memories of that future (that will now never happen from John and Sarah's perspective). All they did was create a new timeline that runs parallel to the original one.
I tend to prefer that theory, at least from a storytelling perspective because it explains why time travelers don't ever seem to lose their memories of the future that they changed after they've changed it. If Marty changed the future so that his dad becomes a successful businessman, where Biff waxes his car, why then does Marty still remember the old future where his father was a pushover who worked for Biff? If he prevented that future from ever happening, why does he still remember it as though it did?
If we apply the Terminator, branching timelines rule to Project Almanac, then nothing should disappear at all (including David). The problem with the movie (and the actual plot hole) is that the movie can't decide which rules to follow, and is inconsistent in its application of them. If the camera doesn't disappear, then neither should David. And if he does disappear, then so should the camera. Either there is one timeline or multiple, co-existing timelines. Not both!
All generalizations are false, including this one!