We're all missing the point. Pathfinder won't be 4E's major competition. PF is a bandaged version of an older edition. WotC needs to worry about a wholly new, vibrant and different game. I don't know the name of it yet, but I bet it's something similar to WoW and, perhaps, 4E.
I don't think so. It may not be Pathfinder, it may be True 20, or some other D20 derived game like SpyCraft or Mutants & Masterminds repackaged into a more generic or FRPG friendly form, or all of the above plus more, but 4Ed's stiffest competition will be some form of the D20/3.X/OGL installed base.
The #1 correlation between a product's success (any product) is being first to market. If you're the first person or company to tap into satisfying a need or desire, you have a significant advantage.
Not that you don't have to do other things to ensure
continued success...
Here, we may be seeing a competition between #1s.
D&D is the established brand name, the King Kong/Godzilla/Robin Hood/Superman/Dolomite/Samuel L. Jackson of RPG brand names. That's a good part of why it was able to rebound after its decline between the end of 2Ed and the advent of 3.X. Its kind of like how Coke lost market share to Pepsi, but never quite lost its grip on the market as a whole.
WotC's 3.X revision, clever marketing and their OGL tactic made the brand even stronger in some ways than ever before. D&D 3.X is unquestionably the most popular pencil & paper RPG out there.
And now they've created a new version of the game that, in all honesty, isn't to the liking of everyone in their established base. To continue the soft drink analogy- they may have just created New Coke.
New Coke had its upsides and fierce fans...but not as many as Coke had hoped. Eventually, Coke Classic (the original formula) was released...and then they dropped "Classic" and New Coke- while still available- is nearly extinct, and is now sold as Coke 2.
In the war between Coke and New Coke, the market worked to prove that the original was what most people wanted.
Just like that, in time, the market will tell us whether the name "D&D" attached to 4Ed will supplant the popularity of 3.X in the marketplace.