I tried an interesting exercise where I went to all the main sites and looked up the number of members.
Enworld has around 71,812 members. WOTC boards have 362,643 and rpgnet has 53,318. This totals around 487773. Let double than number to include every single D&D forum on the net (a gross overestimation since, besides the three sites mentions all others have orders of magnitud less members), and round it to an even one million.
According to what I have seen posted here by and mentioned by the media there are around 20 million D&D players worldwide.
When looking for trends, you don't have to survey a large fraction of the market to find out statistical trends. For instance, when figuring out the Nielson ratings, they make their projections based on a small random sampling of TV watching population. Studies have found that when you do this, results tend to be accurate within a small percentage (1% - 5% if I remember right). So, that means that to obtain a statistically accurate trend on this, you could survey about a thousand people and get some accurate percentages. Obviously this poll hasn't come close to hitting that mark, so the jury is still out.
I do agree with you about one thing, however. People who post to messageboards are not necessarily representative of the gaming public at large. We tend to be better informed about products, more active as players, and more fickle when it comes to products. For that reason alone, this poll might have no bearing whatsoever on the actual reality of the situation.
Now, speaking anecdotally, the trends of this poll do match what I've been seeing locally. Roughly half the people I talk to have or will switch to 4E while the other half want to stick with some form of 3.5. Even my own group is split down the middle, and one of my players likes to run games, so he'll probably end up running an occasional 4th edition game that I will play in while I run the primary Pathfinder game.
Personally, the only objective paramter I use for how any game is doing is sales. IF sales are good and continue to be good then the game is doing well.
As of today 4E is doing well in this department (very well according to WOTC), if time passes and continues to do well, then that answers the overall question of the lines success.
Sales aren't an objective parameter for judging the success of a product unless you happen to work for the company. If you ask any company how their product is doing, they will always tell you that it's performing at or above expectations when the reality might be anything but that. The biggest way to kill your existing line is to either admit that it is performing poorly or to announce that you will end support for it. On the other hand, if you go out there and rave about how well it's doing, you might be able to increase sales through your own hype. It's all marketing. With 4E, we are led to believe that the core books have sold out, but we really have no idea what the actual numbers on the print run were or how that compares to previous editions. Right now it could be the best selling edition of D&D ever, it could be the worst, but most likely it falls somewhere in between.