Just because a fall could have been avoided doesn't mean it's tragic. All falls can be avoided, for that matter. What makes it tragic is the character's intentions and personality beforehand that becomes twisted as a result of his/her actions and experiences.
I'm finding it very difficult to phrase this diplomatically so forgive me if I just lecture:
In literature, a tragic flaw is a character trait that ultimately leads to a character's downfall. Oftentimes this trait is one that would be positive in any other situation. The example through which I learned the concept is Chiuna Achebe's Things Fall Apart (intro to Lit ftw) where a strong, honorable family leader is utterly bewildered by a new world order (colonialism). The book talks about a lot of things, but the basic plot is that the main character would be a fantastic leader under the old ways, but that strength undermines his position after the Europeans show up.
Anakin is in a similar situation. He is strong-willed, loyal, and intensely uncomfortable with dishonesty. These are all traits that can be positive. However, his journey finds him in places where those are all liabilities. Thus, the basic criteria are fulfilled for a tragic flaw.
So, I believe that I have clarified my use of the term "tragic," with which you initially disagreed. It now falls to me to defend the idea that:
[list=a][*]Qui Gon Jinn would have done a better job than Kenobi
[*]The relationship between Jinn and Kenobi imparts some level of irony to Kenobi's failure
[*]The Star Wars saga is more compelling because of [a] and [b]
[/list]
I obviously cannot prove conclusively that Jinn would have done a better job. However, I would like to point out that the series of YA books chronicling Obi Wan Kenobi's life as a padawan paints him in much the same light as Anakin. An impulsive, reckless padawan ignores the rules of the Order to do what he feels is right, ultimately resulting in his expulsion/departure from the order. Yeah, Kenobi quits being a Jedi for a little while. Crazy, right? Well, Jinn is able to guide one padawan through the darkness. [I also have a half formed point about the unifying versus the living Force, but I can't remember which one was Jinn's favorite.]
Jinn's experience and wisdom should be sufficient to allow him to retain the authority position that Kenobi relinquished the second he began thinking of Anakin as "brother." The novelization of RotS calls Kenobi "the perfect Jedi" but he has one attachment: Anakin. Jinn would have maintained a more traditional Master/Apprentice bond, simply because of the age gap.
So, in my mind at least, there are a few reasons to believe that Jinn was capable of doing a better job than Kenobi. Now to the ironic part: Kenobi was at least partially formed by Jinn's teaching. That is to say, parts of his personality and mentality were comprised of what he learned from Jinn. It is a bit ironic that where Jinn succeeded (specifically, keeping his padawan in the Jedi Order) Kenobi failed spectacularly. Of all the skills to pick up, that seems like one of the most important. "Hey this is the Chosen One. Make sure he's a Jedi. don't let him quit."
Last letter in the list:
Those are the reasons I think (a) and (b). In my mind, the tragic loss of Qui Gon Jinn is the only plotworthy element in TPM. Thus: (c).
Edit: didn't proofread this; it took forever to write. Forgive me if it is incoherent.