Two Questions

I'll keep it brief cause there are a lot of question threads. I've finished all three books and I have two questions:

1. Why did the Trisolarans lose interest in humanity after their planet was destroyed? Why would they just leave?
2. When Singer uses the dimensional weapon to reduce the solar system to the second dimension, that will inevitably collapse the entire universe into the second dimension, correct? Does that mean that Singer's species was prepared to downgrade safely into the second dimension? And why would Singer use that weapon on humanity, such a small species? I understand that given Dark Forest Theory, you destroy anyone else you even become aware of. But it seems like overkill to use such an intense, universal weapon to destroy one minor civilisation.

I'm sure these were answered in the books, just looking for clarification. Cheers.

EDIT: Thanks everyone. I forgot that Earth's location was also revealed. I suppose that's the whole point of mutually assured destruction.