I'm honestly not sure how the fonterra this would work. Farmers own shares based out their milk output. They are required to own a certain number of shares based on this. If lab grown milk starts getting processed through fonterra, wouldn't the owners of the lab buy the fonterra shares? If fonterra owns the lab, do current dairy farmers just transition to being owners that aren't contributing milk?
And in regards to the towns, it's worth considering the world is a different place than it was 100 or even 50 years ago. Forestry workers can drive from a bigger city, do a days work, and head home. They don't need the small towns like they used to.
I think anything is just speculation, you can't normally predict the effects of huge change ahead of time. But it will be an interesting next few decades.