In my humble opinion, once the original creator is no longer involved then nothing from that point on is canon. I'm not including anything comic book related since that's a bit of a different animal.
I agree with this sentiment. The ideas are from the original creator so one s/he steps away from the project, there is a lack of legitimacy in any follow up project. Maybe if the new director studied intensively under the original do knows the lore and ideas inside-out and is given the original creator’s full support and blessing, then maybe but still a big maybe.
And I don’t care if some of Lucas’ original ideas were in the ST which is a point that will come up. It has to be 100% Lucas, warts and all.
The best recent example may be A song of ice and fire. The series still has fans despite GoT’s massive blunder of an ending because D&D’s ending is not canon. Fans want to see Martin’s ending and even though D&D apparently heard of the ending from Martin and thus their ending has the elements or final pieces, it is still not considered canon or the true ending because they came up with it and set the path toward it. Even if D&D ended GoT spectacularly, I doubt fans would still take their ending as canon because it’s not 100% Martin.
another good example is dragon ball. GT is widely considered non-canon by fans because Toei made it without the original creator (I think he added some designs only). The new Super cartoon is considered canon but not the manga because the cartoon was made by the OC. Even though Super has a mixed reception, people arnt debating its canonicity because the OC was involved in its creation (they are more blaming his poor memory lol).