I make no claims on what others can and cannot consider "success". You make your own determination. However, I wouldn't fault someone for feeling that until Paizo cuts it chops on something entirely new, their own, then they aren't quite there yet.
I'd make an analogy to an author who writes only "shared world" stories, or only works with public domain worlds. They may be good writers, but until they do something uniquely their own, they're still not quite top-notch.
I would say that a lot of companies do quite well using shared content in MS-DOS even if they haven't created their own language.
I'd say Paizo isn't working a 'shared world' analogy. What the OGL covers is rules mechanics and a few stat sets for monsters after all. Instead of a 'shared world', think of it as a shared language. They are just using open source gaming code to underlie their quite original storylines.
I think, eventually, they'll need to acknowledge and jump it, or stumble and never be a major player in the race again. Their starting situation was unique, but from this point on they are subject to the same market forces as every other game publisher out there.
Eventually, the market will be saturated with Pathfinder materials. That's the reality of the niche. They will not be able to sit on 3e's laurels forever if they want to maintain their market position and size as a company.
I think they aren't sitting on 3e's laurels. They took the gaming rules covered by the OGL, adjusted them to eradicate a few of the worst 'bugs', and published them as an underlying technical manual to play their game. Now they are expanding on the OGL-derived code set with all new classes and rules, which, although new and unique, are still 'backwards compatable' to OGL material with a few minor tweaks. They also have kept 'their' version open source to encourage 3pp participation.
Also, Lisa Stevens has said that she wants Paizo to remain small and nimble. She has been employed by several of the Big Guys and endured their growing pains, and has seen what happens when a company gets too big in this field.
Will the market be saturated? Who knows? There's obviously plenty of room for expansion now. I don't think they will ever devise some new 'D13' mechanic just to have their own proprietal system, there's no need. As I mentioned in another post, the OGL is there, and it is a proven robust ruleset. They are doing what they love, making great stories. They are also free to cross-fertilize with the best ideas from 3pps past and present. They often use the monsters and templates from the
Tome of Horror series and the
Advanced Bestiary in their Adventure Paths. I'm sure as some of the other new publishing houses like Rite Publishing, LPJ Design, Open Design, and others begin to gather steam they will all riff off each other's ideas.
Paizo has a lot of veterans working for them. They've seen the pitfalls of too much splat. They've seen corporate bloat. I think they are in a sweet spot where they can craft the stories they want and remain masters of their own destiny.