This is a classic used from the schoolyard to the boardroom. One of the ultimate shut downs and idea killers “So what, who cares!” Can be worse than a good dose of Strychnine to put thoughts and ideas to death in a business environment.
This statement is flippant and demonstrates deep ignorance of the social conversational backdrop they may find themselves in.
Here is your counter measure for this. Mirror and repeat back “who cares, who cares” in a even monotone voice and then ask “what motivates you to say that?”. This should stun or at least befuddle your counter party and buy you some time to calibrate more questions. Here is they key do not engage in a debate. Stay just out of striking distance by asking questions and validating and labeling what is given to you.
For more on this technique read Chris Voss Never Split the Difference.