And considering how the SAMCRO Prez actually did exit the Sons universe, not to mention everything that happened to him from the Tara-forking Season 6 finale onward, it would have been far more shocking had the British actor quickly and painlessly left Jax in his side-view mirrors.
I fully understand how Charlie Hunnam would have a tough time letting the grim and tortured existence of Jax Teller out of his brain once Sons of Anarchy wrapped.
Even after filming was done, there were still the TV airings and the promotional campaigns, so there might not even be a specific point when Hunnam started to let his inner Jax dissipate.