Possible? Probably--depends on the writer, of course.
Desirable? Potentially--depends on the group.
It's...hard to really say anything more than that. A good writer can make NPC stories quite compelling, especially if those stories actually react to the choices the players make. That can be quite hard; BG3 gets away with more than many tabletop GMs could because there are restrained choice points and fixed dialogue options. However, it's still
theoretically possible that a good writer, composing an overall compelling journey, could include NPCs that are just genuinely really well-written and interesting, with enough advice and support for GMs that the players can go pretty far afield without "breaking" the good writing of the characters.
However, even with all that, some groups just don't care about NPCs. I'm not the kind of GM that can run stuff for groups like that, but I know they exist. Some players chafe at the knowledge their future is even minimally pre-determined. Some delight in forcing things off the rails, whether or not there even
are rails (like cats with objects near the edge of a table). Some appreciate good writing, but don't find NPC interactions that compelling. Etc.
I'm dead certain that there are groups out there that would love this. I'm also certain that this could be used to bring some new folks into the hobby. The problem is, would the folks interested be enough to achieve critical mass? That's the real sticking point.
Branching off of
@Lanefan's answer, I suspect the best thing to do is to write several such NPCs...and have them be genuinely optional as "companions," but with a divergent story if they
aren't recruited. E.g., to use his Little Joe the Fighting Farmboy example, perhaps LJ joining the party means the players get to shape how this nascent godling develops--but if they
don't, then there's a whole different path where now they have to
convince an incipient god to support them. If they
do, this period is instead an effort to help LJ get the power he needs to do something plot-important. The recruitment path results in a deity naturally inclined toward the players' interests, possibly even toward the alignment(s) of the PCs that he looked up to or who he feels "helped" him (note that this doesn't have to mean those who were
nicest to him--perhaps the party teaches him to "toughen up" and he values that lesson.) The non-recruitment path results in a deity skeptical of the party, remembering how they overlooked him in the past--one who might be naturally unwilling to help them with what they need, but who has earned his power all by himself, thus needing convincing, not enhancing.
Such things are gonna be
doubly tricky, since they effectively require you to write two different, parallel, more-or-less-equally well-written stories, when just one is already a tall order, but it again theoretically can be done.