I don't think in any of your examples people had scientific reasons to believe those things were impossible. That's the main sticking point with FTL - it appears to break the universe, rather than just be very difficult to do. It is the 20th century version of reanimating the dead. The "flying machine" for us is a reactionless drive - something that could propel a sublight starship without a need for more fuel than it could possible carry. We don't know how to do that, but it is at least a possibility.