After verifying through deeper inquiry if one finds the world is the reality within the illusion then he cannot again say the world is not an illusion.
If one is frightened to accept the world is an illusion (waking) then he is unfit to acquire Self-knowledge or Brahma Gnana or Atma Gnana. Thus who stuck with the reality of the world, are stuck with the reality of the individual experiences of birth, life and death, which takes place within the unreal world. Thus the pursuit of truth is for those who have courage to accept the reality as it is, that is the reality without form, time and space.
According to Adavaita Vedanta, the Veda addresses itself to two kinds of audiences - the ordinary ones who desire the transitory heaven and other pleasures obtained as a result of ritual sacrifices, and the more advanced seeker who seeks to know Brahman. Thus, the purva mimam.sa, with its emphasis on the karma kanda of the Vedas, is meant for the first audience, to help lead its followers along the way. However, the Vedanta, with its emphasis on the jnana kanda, is meant for those who wish to go beyond such transient pleasures.
Sri, Sankara says in Brahma Sutras: that Brahman is the cause of the world, whereas in Mandukya he denies it. This is because he says that at the lower stage of understanding, the former teaching must be given, for people will get frightened as they cannot understand how the world can be without a cause, but to those in a higher stage, the truth of non-causality can be revealed.
Those who are frightened to accept the world is mere illusion should never indulge path of wisdom.