Definitely only time will tell for certain, but I think that just from comparing microsoft's and apple's histories, and how their various products have fared over the market, I think it's pretty certain that given another 5 years, the zune will be #1 on the market.
That's just what happens when microsoft gets involved. First apple comes up with a brilliant idea (an OS with a GUI) then microsoft gets involved and carries it away, (windows). Microsoft adds more stuff to it and makes it buggier and cheaper, and then owns the market, not because they have a solid product, but because they have a cheap product. I think a similar thing will happen with the zune.
But that's just my opinion.
As you said, only time will tell for certain.