It's the 'dawn' of a new day
In general, when speaking about a sunrise, we refer to the advent of a new day. In regards to this situation, Joshi is saying that things are about to change, big time.
Here's exactly what Joshi says:
Maybe it’s only me can see the sunrise
here. This breaks the world, K. You
ever seen scorched earth? It’s glass.
Shreds your feet. Nothing grows.
-Final Shooting Script (undated), Blade Runner 2049 (2017), Fancher, H. and Green, M.
You see, up until this point, the world has operated in a certain way: the Humans create the Replicants, thereby keeping them as their subjects. This in turn keeps the balance of the large corporations in charge. If suddenly the Replicants can create themselves, and not only that, but start to demand rights, this whole enterprise breaks down.
The world’s built on a wall that
separates kind. Tell either side there’s
no wall -- you bought a war -- or a
There’s an order to things. This turns
it. That’s what we do here, we keep
K even says it himself:
I've never retired something that was born
To be born, is to have a soul.
This removes the separation between Humans and Replicants. It's a huge deal in this world, since so much revolves around the concept.