Han passing the torch. THEORY

In a very rough way:

Finn is the new Han.
Poe is the new Leia.
Rey is the new Luke.

In each case, the main characters borrow from other characters:

Poe has Leia's dedication to the New Republic, but he also has Luke's fantastic piloting skill.
Rey has Luke's aptitude with the Force, but like Han begins the film with no sympathy for a common cause and no concept of family or community.
Finn is the new Han, but like Leia has an aptitude for strategy and an undiscovered gift with the Force.

Harrison is given a swan song because both Larry and Harrison believe its the most heroic thing that can happen to Solo.

Harrison's role is to assure the new characters they can achieve great things with just their wit and and their skill, then to demonstrate that he will sacrifice himself for their cause and in doing so, galvanizing the heroes' commitment to that cause in the process, just as Luke is galvanized twice by death of family: once when Aunt & Uncle are found murdered and once when Ben sacrifices himself for the greater good.

 

I would flip Poe and Finn. I have always thought of Poe as the new hot shot pilot, who is a little selfish and can go off on his own, perhaps against his superiors. Finn to me is more the Leia role, getting rescued by Rey from a predicament on Jakku, but ultimately beginning to hold his own and take initiative as the movie goes along.

I actually think TPTB have kind of jumbled up all of the recognizable original-three characteristics and placed them in the new-three without a direct correlation. In other words, make them recognizable hero characteristics while allowing the new characters to stand on their own in a way.

 

Confirmed! Poe is the new Lando.