If you will not fight this dharmic (righteous) war, then you will be incurring sin, as it will mean disregarding your own duty and fame.
Krishna adds what would be the outcome if Arjuna did not fight the war, instead left it altogether and escaped disreputably.
Any instruction becomes effective only when the advantage and reward for its compliance are spelt forth. Simultaneously the consequences of non-compliance as well must be made clear. Both together persuade the mind effectively!
The outcome for not heeding the call of war is extremely threatening, warns Krishna. War is only instrumental. The actual focus is Arjuna’s mind.
The question is more of forsaking svadharma, Arjuna’s own inborn duty. His widespread repute would also be at stake. Both together would lead him to incalculable sin, meaning calamitous consequence. It will affect Arjuna and his team, as also the people in general, who have great regard for Arjuna. Should he not consider all this and arrive at the right decision?