I don't see Harden as a good fit with the Celtics. We are a motion, passing team with multiple wings and no real offensive big. I can see Harden working well in Philly with Embiid and in Mil with Giannis though.
In 2017-18 the Rockets were up 3-2 in the WCF over the Warriers when Chris Paul got injured. GSW came back and won the series and then swept the Cavs in the finals.
That Houston team had Paul-Harden-Ariza-Ryan Anderson-Capela as the starting core with Gordon, Tucker, and Luc Mbah a Moute as the bench depth. Old friend Gerald Green also played a key role.
If you look at that team, Chris Paul deferred but still had decent numbers. None of the others were "need the ball in my hands" guys. Capela just dunked, Ryan Anderson just stood around and shot 39% from 3, and Ariza is a complementary player.
I believe that you can build a title contending team around Harden as this did in 2017-18 (until Chris Paul got hurt).