Bannon was adored by the far right and loathed by many in the center and left and parts of the right because of his ties to the overtly racist, anti-Semitic, and anti-LGBT website Breitbart, which he formerly headed and to which he will return. Breitbart has been a critically important grass-roots support base for Trump.
At the moment, competing narratives exist in the mainstream media but are primarily of two sorts: (1) Bannon was fired and (2) it was an amicable departure. Time will tell which of those is correct.
If the Trump administration and Bannon are not at war from this point forward, then the White House status quoa probably will not change much.
But if this departure signals the beginning of a war between the Trump White House and Bannon's minions on the far right, what little support Trump has will continue to dwindle.
Bannon's exit may cause Trump's base to fracture and collapse, dividing into two camps, with each one smaller and less powerful.
If that happens, then Trump will be falling into the Richard Nixon basement. His departure from the White House will be sooner rather than later.
Alternatively, Bannon may remain loyal to the President himself but try to steer his administration from the outside through Breitbart and other far-right media ventures, making war on key individuals in the West Wing. That doesn't sound like something that would stablize Trump's White House and lead to a positive end.
All of this is speculation right now. It's too early to tell what the near-future will bring. Whatever the answer, we are certainly living in interesting times.