Not as sad as King Kong but.....

The ending was NOT that happy if you really think about it. I know nobody lives forever but people with giantism usually do not live very long and have horrible health problems towards the end of their shortened lives. Andre the giant is a great example. He was to a human what Mighty Joe Young was to gorillas. A healthy human can easily live 90 years and sometimes past 100 but Andre died at age 42 (my age actually) and had painful health problems for the last few years of his shortened life.

Being so much larger than a regular gorilla would cause Joe to live a much shorter life than the average gorilla, maybe only half or less as long and have major health problems for his last few years. He looked very healthy at the end of the film but had the story continued he likely would have started getting health problems and die within the next few years.