Having experienced first-hand the way the IPL started out I have to say that it was in response to what the ISL was doing and it was also done to generate some serious money as well which I suppose goes along with the destruction of any sport not just cricket. It did die down a bit somewhere towards its third iteration but then it picked up again and got really good. I wouldn't say it destroyed the game more-so moved it in a different direction that people were used to seeing.
T-20 is a far easier event to watch just because of its explosiveness and the way teams approach games. Does that have an effect on the other styles of cricket? Yes. Does that necessarily mean its destroyed the game itself? That's really up for interpretation. Having more facets to a sport could dilute the others based on its popularity but that doesn't meant an ODI or a test-cricket match is suddenly going to get worse does it?