I think this idea is made even more stupid (and hopefully unlikely) simply because enjoying the journey, having fun, and the meaning of friendship etc are all things the main characters are already good at. If One Piece were really just some lame corny message like that, it would be terrible overkill. The story has been teaching those messages from Chapter 1 in a non-lame way, why ruin it?
For the ending to be good storytelling, One Piece (or at least the last leg of the journey) would have to be something that really challenges the characters with things they are not good at handling. I don't know what that will be, but I'm expecting more emotional trauma with a similar impact to Ace's death.
I don't think the Rio Poneglyph is the One Piece, we shouldn't know what it is at this point… but I'm still excited to find out the real history!