I'm actually doing the their fall/metamorphosis in two parts essentially. In the hundreds of years before my prequels begin, the Jedi for all intents and purposes have dispersed throughout the galaxy. The core motive was moving away from directly working with the senate due to its growing corruption and a belief that they could better serve the citizens of the Republic by offering assistance all throughout the galaxy rather than just the core worlds.
This would create two side effects. On one hand it allowed them to grow beyond being just knights. Jedi healers, engineers, philosophers, teachers, farmers, now started becoming the rule rather than the exception. Countless users of the force all doing their part to assist without ever using a lightsaber. They had their own first steps into a larger world.
However this also led to them in many ways becoming mythic rather than public figures. Jedi knights of the past were a highly organized force that were discussed and seen as the guardians of peace and justice that the old days essentially needed. As the centuries went by however and their efforts were put towards more indirect areas, what Jedi were observed seen more exceptional than extraordinary.
This evolution is at the center of a conflict between Kenobi and the only other active student of Yoda, Nellith, both of who represent the two sides of what the Jedi are to become in light of the Clone Wars. Obi-Wan believes that even though they are already serving in countless support positions, for the sake of winning the war, Jedi need to go back to the old ways and have knights at the front line. To Ben, while he understands their position, the Republic has to come first.
For Nellith, serving as a quasi-defacto leader of many Jedi, putting them at the front lines offers too many risks. Namely that Knights are the most likely to be tempted by the dark side, and that turning what have been peace time Jedi into soldiers on the frontlines could create enemies worse than what they're fighting. In addition the possible risk of close organization making them easier targets weighs on her mind. There's also a subtext that she has grown to truly value what the Jedi have been allowed to become and doesn't want it to disappear in the fog of war.
In this case both are right. The Republic did need the Jedi at the front to win the war, but that very organization after Episode II will help Palpatine in wiping many of them out and prove Nellith correct. In the end while Kenobi's move wasn't necessarily the wrong one, his failure with Anakin ensured that which helped to save the Republic would serve to doom the both it and the Jedi.
Ultimately its about expanding the nature and character of the Jedi beyond simple white hats and to further demonstrate what a tragedy their destruction was. It wasn't just the lives lost, but that the new essence of what a Force user could be was annihilated, reducing a vast amount of all kinds of Jedi to much fewer in both kind and number.