Objectively, isn't this how Pathfinder should be looked at? Shouldn't the question be what about Pathfinder makes it necessary to abandon 3.5E?
I think there's another aspect to keep in mind. Months ago, initial information was released by Wizards about licensing agreements to publish 4.0. Third party publishers were not allowed to continue publishing 3.5 content when they begin publishing 4.0. Also, I'm not sure how long Wizards would continue to license 3.5 D&D products. I'm not sure if Wizard licensing stance has changed in recent months, it's possible.
So, it seems to me that Wizards has abandoned 3.5 D&D, which means the rest of the community (Paizo, you, me, other third-party publishers) have interesting and difficult decisions to make. Sure, individually we can continue playing version 3.5, our books will not disintegrate overnight. Still, publishers will need to produce and sell new product. New players will need to obtain the base rules system. Also, current and future OGL content will need some sort of rules system to be available.
The Paizo boards had several postings related to the business decisions regarding Pathfinder. As I recall, another benefit is that Paizo could publish "4th D&D" licensed product at the same time as they release "Pathfinder RPG" content, since they're not reliant on Wizards to license "Pathfinder". I think other third-party publishers have also expressed some plans to publish "Pathfinder" related materials, since they could publish those alongside "4th D&D".
My input to the Pathfinder discussion: I like the new character options, and I thought a lot of the rules were worded more carefully...I hope it will be harder to find examples where the rules are unclear or contradictory. I do hope that they continue to make changes. I would prefer to see more changes toward a 3.75, rather than keeping it very close to 3.5 and ending up with a 3.55. I do hope that Paizo will collect other opinions and rules options to develop some sort of "Unearthed Arcana" style collection of variant rules.
One last comment, though I know this is already a long post, about the DM prep time for adventures. Probably my best comment about the Paizo "Rise of the Runelords" product is that it requires very minimal DM prep time. Several times we've started a session and I've only spent 30 minutes reading over the next few encounters. I don't believe this is because I'm some sort of master-DM, rather that the material gives a good understanding of the storyline and the enemy's motivations and goals.
Regards,
Charles