Hmm I'm not so sure. I don't think that in 30, 40 years we'll look back and see a massive void around this period of time, where music was rubbish.
I would imagine our tastes in music will have changed by then, and we'll look back and say, well the turn of the 21st century was a time of money-hungry record companies, but it lead to x style of music, which was so much better than the crap being produced today.
I don't know what defines 'good music'. I know what I like, some of it ranging from the good times of the 60s (bearing in mind I was born in the late 80s), right up to some of the cheese and crap being produced today.
That Rihanna song mentioned, that samples the Numa Numa song. Sure it's a shite song, but you can't deny when it gets played in a club it gets everyone jumping. Maybe that's what we, as a population, like these days.
Good music to me is something that sounds good, something that makes you think, and something that puts you in a good mood. Whilst today's rap might not achieve that on all levels, there is still music being produced that does. And it doesn't necessarily fall under the niche market, or the non-mainstream