This "real reason" of which you speak does not sound like the real reason to me. This is the best possible gloss you could put on it, not the most plausible one.

Having an open ulterior motive is fine in my book. Don't trust me? Think that makes the net expected utility too low? Fine, go donate to the Methuselah Foundation or Foresight Institute instead. But don't think you can get away with just donating time. Find someone you trust enough to donate money.