The biggest improvements Tesla needs to make in S/X is battery packs using the 2170 batteries (which could possibly reduce cost or increase range), V3 supercharger support (likely needs the 2170 battery packs), continuing to mature the EAP software, starting to rollout FSD features (in driver assist mode), and adding some form of screen mirroring (Apple CarPlay/Android Auto or alternative).
None of these requires a refresh of S/X.
If Musk/Tesla really believe they will have FSD available in the next several years, it makes more sense for them to wait until they are close to achieving that before doing a refresh, because a vehicle with FSD will de-emphasize the driving function, and likely would increase emphasize on what the person in the "driver" position and the other passengers are doing while the vehicle drives itself (possibly even automatically connecting to chargers). That could require a major redesign of the cabin - and add features like video display while driving, mobile hotspot, …
Until then, it's always possible Tesla will release smaller improvements or some tweaking of the S/X design, though with their resources focused on new vehicles - Y, pickup, semi, Roadster 2.0 - seems unlikely we'll see any major redesign to the S/X in the near term.