I found the ending to be immensley tragic and sad. To me the film ended as sadly as GF2 did, and that was a brilliant movie.

As he said to Kaye at the start of GF3 Michael spent his life protecting his family. He didn't want to be the head of the family but the circumstances of GF1 put him in that position.

After the events of GF2 he tries to redeem himself by legitimising the Corleone Family but after all this, after over 30 years of being a target, after having the lives of his family at risk and then finally getting out of the business he loses one of the most precious things in his life - his daughter.

In the end it was all for nothing, Michael still lost and for all we know he may have even chosen to exile himself from everyone after that moment to live the rest of his life alone.