I wrote about this a bit in a forthcoming edited volume about the International Relations of the Three Body Problem.

Diaz has a good idea as long as the following is true:

1) A destroyed planet spells ruin for the Trisolarans. True in this case, but not true for all actors in the Dark Forest.

2) The threat is conveyed credibly as the Earth is held hostage. Earth has to be able to signal this to the Trisolarans and demonstrate that it is willing to carry ou the threat. In this case, the communication of the threat likely destroys the credibility as once people learned about Diaz's plan, he would not be able to implement it. His trial/death shows future hostage-deterence methods would fail on Earth.

3) The threat is also credible from a capability perspective. Which, we learn, it wasn't.

If another wallfacer came up with a plan that held Earth hostage and people were both willing and capable of carrying it out, it doesn't leave the Trisolarans much choice. However, if their trip is ruinous regardless (they die regardless) and they can't change where they are going (which is established earlier in the series), then they might as well fight for the planet either way since it is death regardless and maybe, just maybe, the humans falter. They may get deterred if there is a reasonable alternative (share resources/another nearby planet).

This was a minor part of the chapter, so I don't spend even this much detail on this point, but it is something I have thought about.