Don't get me wrong, I see the "reasoning" behind why people are jumping to that conclusion. For those disappointed that David has been revealed to be the creator of the aliens, making him turn out to be the space jockey would be an obvious way to twist the knife further. So of course everyone is automatically assuming that this is indeed what will happen in the next movie (that is, if the next movie even comes to pass).
The irony is that some here are already contorting themselves to logically justify this completely unconfirmed plot twist that they don't even want to occur. It's bizarre, to say the least.