Trisolarans knew about the rule "sit silent," so they should not allow Earth to send another transmission. That means they must use sophon to freeze Earth (just like what happened with the Trisol during sophon construction.)
Another bonus would be the destruction of mankind and a fresh empty planet for trisolarans.
This situation occurred in the Trisol system. When it's in its extended form, the sophon is extremely vulnerable to nuclear attacks.
Several brilliant fireballs appeared in different parts of the mirror, their blue color distinct from the light reflected from the mirror. These were the exploding nuclear warheads launched by the Trisolaran space defense corps. Because the explosions were happening outside the atmosphere, there was no sound. By the time the fireballs disappeared, several large holes appeared in the mirror, and then the entire surface of the mirror began to tear and crack, until it had broken into more than a dozen pieces.
One assumes that the defence forces on Earth would try much the same thing (e.g. whack it with nukes to see what happens) if a sophon unfolded in space, noting that it's roughly the size of a planet and hence needs to be pretty close by in order to make any significant difference to the amount of radiation being received by the Earth.
My old hypothesis:
Well, due to the Dark Forest Hypothesis, the trisolarians may fear that an entire planet being wrapped up for a long time may cause suspicion from the technologically hyper-advanced civilisations, causing the Earth and Trisol to be wiped out, like what happened in the third book.
The accident that happened at Trisol was already a risk too large for the Trisolarians to manage.
What I agree with now:
The second book shows that the sophon cannot propel itself or maintain its shape after two-dimensionalization. This makes it vulnerable to ground attacks. The following text is copied from a pirate version.
但他随后得知，智子在二维展开后没有任何空间机动和定位能力，只能以行星的引力为骨架保持形状，如果在太空中展开，将很快在太阳风等因素的作用下失去平面形状折叠起来，这就是二维展开后���智子只能在包裹三体行星的情况下才能保持形状进行电路蚀刻的原因。
The deepl translation is as follows. Sadly, I do not own an English copy of the book.
But he then learned that the sophon had no spatial maneuvering or positioning ability after two-dimensional unfolding, and could only hold its shape with the gravitational pull of the planets as its backbone, and if unfolded in space, would quickly lose its planar shape and fold up under the action of the solar wind and other factors, which was why the sophon could only hold its shape after two-dimensional unfolding to be circuit-etched if it wrapped around the three-body planets.
Actually, Luo Ji was concerned about Trisolarians messing with the broadcast via blocking the sun, so maybe temporarily blocking a planet or even a sun doesn't matter that much.
