Why would you invest in a system that's not going to be supported in 2 years?
2 years is a long time, plus MS hasn’t even properly outlined that titles will no longer release on One X. It still makes the (cheaper) One X a valid choice for many wanting to save £100+, again I’m not sure why you can’t understand this.
The One X is also 4K capable so brings a slightly different feature set to the table as is outlined here in this thread.
Sea of Thieves:
4K / 30 on One X
1080p / 60 on Series S
4K / 30 is actually decent for a machine which is around £150.
The series S is being marketed as a FIFA / COD / Fortnight cheap box but these titles will all still come out and be running on the One X for years to come.