MS can adjust X1X price to maintain healthy, if somewhat niche, sales. All the while it's out there repairing the branded damage X1 Phat caused.
399 by the end of 2018, 299 by the end 2019 with a slim model, at which point MS can discontinue X1S.
They should quietly in-line upgrade the S to something like 2.6 TF, 8GB GDDR5, should be able to maintain current very low Xbox S pricing (as it'd basically tech wise now be a upclocked vanilla PS4, should be easy to do with 4 years of process improvements). IMO do this with not so much fanfare. Announce it at E3, hey it's on sale in 1-2 months. It'd be a great move. With the current modularity around consoles.
I almost think it might happen by next e3.