This is a graph of some QUANTITATIVE RESEARCH what I have done.

The data set is all records to have got into the Top 10 during January-June each year. Genre classification has been done by me - where I couldn't remember a record or artist AT ALL I left it off the graph, which is why the %ages don't add up (this affected about 4 records total). Seasonal records - eg World Cup songs - are also in the %ages but not on the graph. I didn't classify the Elvis reissues in 2005 at all.
Categories are blurry - especially in 2000/1/2 when dance and 'urban' and pop were all quite interrelated. Vocal, song-based garage records, for instance, may have ended up in any one of the three though I tried to be consistent.
Urban includes hip-hop, R&B, ragga and soul - R&B dominates within this from approx. the middle of the decade.
Nu-metal I tended to put into "rock", emo I tend to put into "indie" - this is a deeply arbitrary classification, obviously. There was a definite split between "rock" and "indie" in the past which I think is no longer the case, the two categories are quite overlappy now.
I haven't tried to filter 'teenpop' out from 'mumpop' or any other kind.
With all these caveats I think the graph tells some interesting tales and gives a bit of evidence to ideas that have been floating around here.