The Shopkeeper in Snowdin Town is another survivor

In fact, all shopkeepers survive.
Gerson talks about being safe in shops on the route, I'm sure you know.

Yeah, I used the Snowdin Town one as a representation of the shopkeepers, and as an example of a survivor

What I want to know is why sparing monster kid makes any difference since you end up killing the same exact amount of monsters otherwise.

Alphy commits suicide on most routes that undyne/mettaton dies

Oh yeah, i forgot about that.

However, she evacuates everyone in the full-fledged genocide route, and can't afford to leave them in such a world.

No everyone dies, including humans. It's kinda impossible to destroy the world without that.

but you reset the timeline afterwards, effectively killing everyone else

Well, resetting the timeline doesn't exactly kill the monsters, it rewinds time to when you fall off Mt. Ebott, and it erases the monsters' memories