"How else can the city grow now with the new annexation laws in place."

The new law does not stop annexation. It gives the ones in the proposed area a choice. So to answer your questions the city can still annex land, it's just going to have to truly offer addition or better services that would make >40% of county residents want to be a part of.

Or, they can continue to promise developers any rezoning or conditional use permit they desire in exchange for voluntary annexation.