Currently, there are about 5-6 pools who mine around 75% of the whole Bitcoins. If you target those pools with DDoS attacks, you can make them unstable, failing, etc. The result would be 4 times slower Bitcoin network.

Sure, new pools may show up. It will take some time, though. And they can be targeted, too.

Actually, I think that the above is already being performed by SOMEONE.