Why do the five consciousnesses transform only at the point of attaining buddhahood?

"The sixth and seventh consciousnesses are transformed in the causal stage, while the fifth and eighth are perfected on the fruition stage." This means that the sixth and seventh consciousnesses must undergo continual transformation throughout the process of cultivation, whereas the fifth consciousness and the eighth consciousness are transformed and perfected only at the very end when Buddhahood is attained. The process is the causal stage, and attaining Buddhahood is the fruition stage. In this process from cause to fruition, the fundamental transformations of the sixth and seventh consciousnesses occur three times, while their gradual transformations occur countless times. Why must the sixth and seventh consciousnesses undergo countless gradual transformations before wisdom can be perfected and Buddhahood attained? Because the sixth and seventh consciousnesses are endowed with all ignorance and afflictions, they must continually remove ignorance and sever afflictions through cultivation, continually purify the defiled nature of the mind, and at the same time continually increase merit and pure wisdom.

From this it can be seen that although the sixth and seventh consciousnesses are endowed with the nature of afflictions, they will ultimately also be endowed with the nature of purity, the nature of bodhi, and the nature of nirvāṇa. This shows that the sixth and seventh consciousnesses are endowed with both wholesome and unwholesome mental factors (caittas); neither of them possesses only an indeterminate (avyākṛta) nature. Otherwise, cultivation over such long kalpas would be impossible, and indeed there might be no need to cultivate at all.

Why, then, do the five sense consciousnesses not undergo continual transformation during the process of cultivation? Because the five sense consciousnesses are completely subordinate to the manas; they are tools that the manas uses to discern and process sense objects, entirely serving and obeying the manas, with no independence of their own. Therefore, the five sense consciousnesses have no wholesome or unwholesome nature, do not need to cultivate good and sever evil, and thus their mental nature undergoes no transformation, nor does it need to. However, because the merit and wisdom of the sixth and seventh consciousnesses are not yet complete and their stages are low, the functions of the five sense consciousnesses that accompany them are also low, and the endeavors they carry out are imperfect and incomplete. When the Buddha is about to be attained, the merit and wisdom of the sixth and seventh consciousnesses become perfect and complete, their virtues are fulfilled, and their mental nature is perfectly luminous; the accompanying tools they use must naturally be upgraded to the highest grade, capable of assisting the sixth and seventh consciousnesses in perfectly accomplishing all endeavors without obstruction. In this way, the five sense consciousnesses must be matched and consonant with the virtues of the manas, transforming from the nature of consciousness into the nature of the wisdom of accomplishing what is to be done (kṛtyānuṣṭhāna-jñāna), with exceedingly sublime virtues.