It is unlikely that 3d printing is going to kill off Games Workshop. Games workshop is doing a good enough job on their own killing off their tabletop brand but that's beside the point.
Right now you can buy a set of space marines, get some resin, and duplicate all of the model parts using vacuum sealing for a fraction of the price of GW models, but this hasn't killed tabletop largely because this takes some skill and practice.
Along the same lines, you can't just go out and buy a 3d printer that will duplicate your favorite space marine. You need to go get a higher end printer if you want the same level of detail that you get from the mold-produced space marines.
The assembly line style is going to produce much better, and cheaper, models for the foreseeable future, meaning for the average person it is going to still be cheaper to buy from GW than it will be to buy a 3d printer and start printing out their own army.
Of course in the further future, assuming 3d printing gets really good AND really fast then we might see a change in GW's model. They'll buy some high end super-industrial 3d printers and churn out a lot of models. You'll likely see either larger models, or rules designed to encourage use of lots more models for play. Either way, you'll likely see that it is still cheaper to buy from GW than it is to get your own 3d printer and print them.