I have no doubt the G3 will take some S sales.
I think it's unavoidable, and if that reality forms any part of Tesla's planning process then they're fools. Your next generation product will cannibalize sales, but you're much better off stealing your own customers by giving them a product they like more than letting someone else take them from you. Remember Lotus 1-2-3? While it still ruled the roost Lotus had a remarkable new spreadsheet called Improv that they axed to keep their customers from abandoning their aging flagship product. So their customers moved to Excel instead.