Sorry for a ramble but here it is -
Short answer: it might be (it might never be as popular and is dying a slow public death like jazz)
Long answer - in no particluar order:
1. As all fads come and go, rock is the same - it came and is going
2. Too many changes in the music business (labels are now owned by large conglomerates) has led to a business structure that does not support long term artist development which is critical to a rock career (lost organizational and artistic memory of corporations)
3. Music tastes have fragmented and re gelled in ways that are not easily categorized (stylistic values have taken over artistic values)
4. The growing influence of a 'global music culture' will eventually hold sway (give it a few decades), think of all the stereos and musical instruments being sold in India, Brazil and China in the coming years. In fact, it may the adoption of rock in these cultures that might give it a new life. To them, rock is still a popular and relevant icon of meaning and self expression of individual freedom and urban artistry
5. Development of global popular culture will rely on new media systems to evolve and spread, so if a musical style is not associated with new media, it will eventually be considered anachronistic
6. As the baby boomers pass into retirement, rock might resurge as hobby or past time among this segment. After that all bets are off.
But that doesnt not mean that there are not some really GREAT famous radio-worthy rock bands around today; Radiohead, Sahara Hotnights, The Datsuns, White Stripes.
However, rock is not THE cultural force it was, in say the 1960s and 1970s in the West.