Think about how much people will change, how many lives they will live over 300 years.
Also keep in mind a society that is able to develop and sustain anti-aging medicine has to be by definition more advanced than ours.
Boredom is not valid, since nobody talks about people being forced to live. However, I don't think I would be ever bored. There is so much to do. So much space so many things and so many minds to explore. The tragedy, the insult of the brevity of life outweighs the dangers of boredom infinitely.