Help needed, desperately.

I've been running my campaign for well over a year now (our anniversary just passed) and I've had one too many breakdowns in front of my players when I started to realize the supporting cast was suffering from "bloat".

My first solution was introducing cuts to focus on characters who had more value or contributed more to what the players needed. But that didn't seem to solve much... Eventually, I realized the issue was a lot of my characters suffered from lack of narrative purpose. Mainly because I started off our campaign in a sandbox. Basically an extended session zero which I deemed the "prologue" for the actual story once it was made.

But now I have a lot of "SPCs" who've been left over from that period.

The easiest solution is just to remove characters who aren't necessary for the story, right? Buuuut... That would result in a lot of turmoil since I'd essentially be getting rid of what's practically some of the closest bonds that the players have.

So, I'm really left scratching my head here as a DM cause I can't really proceed forward since thid 50% of my cast I'm just uncertain of in the long term future.

Should NPCs exist solely to service the players or would that be contrived and devalue them as characters and thus the narrative as a whole?

...

Because the alternative seems to be getting rid of them or pushing them so far back that I can inevitably have better characters take their place although it feels as if I'd be killing my campaign before it even starts if I take that course of action.

Edit: This needs to be clarified apparently. I have grouped my NPCs... Supporting-Protagonist Characters are essentially characters who mechanically exist to fulfill rolls within the parties of my cast. By extension they are supposed to have narrative roles.

Edit #2: The game is completely made by me... From scratch with inspiration taken from other systems. Hence the long prologue to essentially figure out what exactly my vision was and what my players wanted.

...

Final Edit: Thanks to everyone who commented down below! I'm resolving the issue now with my players. You can still leave comments if you want but I'm gonna handle it from here.