Unless the new game looks like shit I wouldn't think about dropping the series. I have a hard time believing KH3 will be somehow less enjoyable than like Days or UX and I am clearly still a fan after playing those. Its more like repeated disappointment that makes me drop things. I am actually interested to see if they can stay away from some of the issues that will be rectified with a "clean slate" for the main plot.
And tbh, until something else brings out a Disney RPG I will probably always be drawn to the series lol.
But yeah, this whole "oh after this, I am done" thing is foreign to me, I could never do that. Its always an actual sense of being dissatisfied that makes me want to drop something. And other times I take small breaks from shows or games, and then catch up when I am in the mood. Like, a series like Pokemon that releases a new game almost every year, and like KH did for quite a while. I really don't care to go back to that, in hindsight. I like having some space between releases, it means I don't get burned out. I think thats why I don't call for games to just end like I do with TV shows or movies. But, with large story driven games like these, I do think it does need to be hung up eventually.