Hey Viraj,
Interesting thought!
If you look theoretically, yes Hadoop and Mainframe are in opposite directions but it doesn't translates into that, it would replace the mainframes. That's not going to be done any soon, in distant future it may be possible. The rationale behind it are two points:
1. Hadoop right now is fresh and new and has to prove its effectiveness in many more use cases before it can be trusted by organisations to leave their existing reliable software architecture on mainframes and take a risk on moving to Hadoop.
2. Commercially, it is not viable for any organisation to find budget to move its software architecture to Hadoop in short time. There are huge budget costs involved, so much so that no organisation would shift immediately, unless it sees huge and sure business advantage.
That was a nice thought, Viraj!
Best,
Nitesh