What did Migi do in the last episode? I remember him saying something about walking on a different path so he would sleep for a long time and that he possibly won't wake up anymore. He could've stayed in Shinichi's hand as he normally did and continue to coexist with humans like Uda and Jaw. But why would he do that? Was it for his own good?
During his stay in Gotou's collective, Migi was put under hibernation, but with a constant and pleasant flux of information sent by Gotou through Parasyte telepathy.
However, you are mistaken. Migi did not go anywhere. He remained as Shinichi's right arm. Since the beginning of the series, Migi can delegate control of his muscle structure to Shinichi through the nerve connections of the arm stub. So Migi was to be forever as Shinichi's right arm/hand, hibernating.
This connection is telepathic too. Since Parasytes have empathic telepathy (they can convey feelings) and it is shown that Migi can enter Shinichi's dreams, it means Migi and Shinichi share some thought bond, probably through the nerve connections (Shinichi mentions there are Migi cells in his brain since the chest wound).
This could explain why
Migi temporarily awakens when Murano falls from the roof, extending the arm and lifting Murano back to the roof. Then he tells Shinichi "you can do the work of holding her now" before going back to hibernating.
Keep in mind that the conversation above happened telepathically.
Migi tells Shinichi that it was so pleasant staying in the Gogou's information highway that he would not mind remaining part of Gotou's collective hibernating forever.
After he was reunited with Shinichi, he had a ton of issues in mind. This should be a good point to stress that Migi was a scholar type of Parasyte, similar to Tamura Reiko. So to him, understanding the information was more important than merely subsisting.
Several of the issues that Migi wanted to solve:
- Reiko's signal when she died protecting the baby. Migi was bedazzled at this brand new kind of signal. We can think it was maternal instinct, the will to put another's life above one's own (which is unthinkable to Parasyte mindset).
- Gotou's information torrent. You can think like Migi downloaded several Terabytes of data from Gotou while hibernating. He wanted to process that information.
- Give Shinichi a normal life. By the end of the series, Migi is no longer of that Parasyte mindset, just like Reiko. The fact that he was willing to die in order to allow Shinichi to escape Gotou proves that he too was placing another's life before his own.
- Lay low. Since every Parasyte that knew Shinichi/Migi was dead, and all the others were deadly scared of humans in general after the mayor's office carnage (going so far as to change their diets or to stop killing in an obvious way), there was little probability another hostile Parasyte would attack Shinichi. Meditating would turn off his signal, making Shinichi just a normal human, and giving them both peace of mind.
- Meditate on the question (the one with the answer 42). Without any imminent threat, having learned almost everything, and with his nutrition granted by Shinichi's body, life for Migi was bound to be Boring with a capital B. So he turned to the higher thought processes to make up for the lack of more physical endeavors to pursue.
In the last episode, Migi shut off all external interference to his thoughts, so he wouldn't talk, hear, or sense anything. Instead, he would only focus on the information he had collected up until that point, and this "hibernation" would allow him to contemplate it without being disturbed. Migi said that there was a possibility that he would awaken from this state one day, but that he might be dormant for the remainder of his and Shinichi's lifetime.