It may be wishful thinking but I just don't see Elon ceding the market to the Taycan (I understand some believe the Taycan is closer in size to the Model 3 but pricing is certainly closer to the S). The Model S is Elon's baby - it is what brought the company to this point in time to where it has the ability to dominate the BEV market. With the recent announcement of the Taycan having 30,000 reservations, I have a hard time believing Tesla will allow this segment of the market to slip away from them.
From a strategic standpoint, wouldn't it make the most sense to allow Porsche to fully announce the Taycan (with all of its specs and pricing) and then shortly thereafter announce an update (not a "refresh") that makes the Taycan look like outdated technology. With Porsche's rollout of the Taycan due in September, the end of the federal tax credits on December 31, moving the battery investor day to early next year together with CraZ8's info indicating an update has been moved to the first quarter all add up to something significant happening in early 2020 for the Model S.