#2 The East is a much weaker conference
The Eastern Conference has drastically improved over the last 12 months, but the division still trails the West by a long way. The Milwaukee Bucks are still too dependent on the Greek Freak, Toronto looks likely to lose Kahwi Leonard for free this summer, and while the Philadelphia 76ers provide a credible threat on paper, they have still yet to make a dent in the playoffs.
With the current Celtics' roster, Irving faces a tough task to dethrone the dominant Warriors, although he currently has a path to an annual NBA Finals appearance.
Staying with the Celtics provides Irving with the opportunity to replace LeBron James as the face of the Eastern Conference. The 27-year-old would also have a shot at becoming the regular captain of the East All-Star team.