Well, I think 4E wioll stay on top, for the reason it is the face of the hobby for new players. It is in bookstores and gaming shops. A lot of people play it, and it is shiny and new.
I see Pathfinder like I see Iron Kingdoms nad other 4rd party rulesets in 3.X. They will ahve soem dedicated fans, but instead of doing work that is of direct benefit to 3.5 people, it is a bit off toteh side, a bit hard to convert and use. And that will limit them, and there will be a 3.5 rift with WOTC 3.5 on one side and Pathfinder 3.75 on another. That will hurt Pathfinder. Still, it will probably hurt them less than a total new system, which would be an utter failure, in my opinion. The bottom line is Pathfinder is not 3.5.
WOTC has done a good job of driving 3pp from 4E, and will probably continue to do so. Thus ther ewill be fewer places making up stuff for current gamers. Maybe if the current 3.5 crowd gets mroe vocal and supportive of people making product for them, it will change, but I jsut am not seeing it, and have not seen it in the last year. I play 3.5, and I have seen nothing come out that i would seriously consider buying.