Explanation:

The same Brahma Loka is said to be eternal for some other souls, who do not return back. The priests say in the beginning of the ritual that they are praying for the departed soul to reach Brahma Loka forever (Shashvata Brahma Loka Nivaasa siddhyartham…). Moreover, Krishna says that souls reaching His abode do not return back. You cannot say that Brahma Loka means the abode of the four-headed Brahma, the Creator, and hence the souls may return back from such an inferior world. In such a case, Vishnu Loka and Shiva Loka must also have the same fate since Veda says that all the three are one and the same (Brahmacha Narayanah, Shivashcha Narayanah). If you say that Vishnu Loka or Shiva Loka is eternal, Brahma Loka must also be eternal, since Krishna said that He is the Creator, Ruler and Destroyer of the world (Aham sarvasya Jagatah…, Mayi sarvamidam…—Gita), His abode must be the same as that of Brahma, Vishnu and Shiva. Since Krishna said that His abode is eternal, the abodes of these three must also be eternal. In such a case, there is a contradiction in the Gita, since Brahma Loka is eternal and also non-eternal.