I understand your point and agree on some of it, but I also think you are missing a few things, too. Fitz has played largely mediocre to great. He has a great personality and knows how to lead, he is tough, and easy to like. I am happy he is on the team and believed he deserved to start the season. All that said, while Fitz is winning, that doesn't mean he is the best option. Against anyone but the Jets, no 3rd down conversions for an entire second half of football and two picks just doesn't cut it. In every game but the jax and 49'ers games, there have been plays left on the field, and yesterday's second half is not indicative of a team that can win a playoff game.
Tua is young, but appears to be every bit as likable a guy as Fitz. Is he a leader? That has to be proven on the field. Tua offers the possibility of making more of those plays that are currently being left on the field. Most rookies struggle, or are streaky, but Fitz is already streaky. Is it really as big of a loss to move Fitz to the bench and Tua onto the field as you think it is? Yes, he could be awful, but there's no rule that says once he starts he can't be benched if necessary, either. If he performs as, or almost as good as Fitz, have we really lost anything? I think there is more to it than just who is winning at the moment. Remember when Griese got hurt and Morrall led the team team to the playoffs only to get benched when Griese was again healthy? Shula wanted the most talented QB in. I know it isn't a truly comparable situation, as Griese was already accomplished, but still, there is something to be said for going with the more talented player.