This, and additionally from what I've heard Tesla hasn't fulfilled the order with Panasonic yet to buy 2 billion 16500 cells yet.
About the fit/dont fit: Could be, the 2170 is just 5mm higher. so maybe this could work in the current pack. But still, as you point out too there is a redesign in the works. It doesn't make a lot of sense to work out a new pack design now if you're going to redesign the entire car a year later.
I suspect the 2 billion cell contract is pretty close to filled. If all packs were 7104 cells (original 85 and 90 pack), 2 billion cells would be around 280K cars. Now a fair percentage have been smaller pack cars that used fewer cells. But even if all were 75s with 6216 cells, that would be around 321K cars. They are just approaching 200K sold in the US and I think about 60% of their total sales have been in the US. Some of the cars in that total are Roadsters and Model 3s, but the vast majority are cars with the 18650s.
If they haven't used 2 billion 18650s yet, they are pretty close.
With a redesign of the S/X, who's to say they won't reuse some elements from the original design? They could stick to the same skateboard, probably with new motors, and even keep some other elements of the original car. The only real drawback of the Model S is the roofline slopes too much in the back making it kind of tight for taller adults and the interior was always rather spartan. Adding more storage in the interior, tweaking the roofline angle a bit, and then new motors and electronics underneath would answer most complaints about the S. An S wagon would be awesome. Robert Llewellyn's Fully Charged showed a "shooting brake" conversion that was much more practical than the current S.
In an X redesign, they should offer a version without the falcon wing doors. They could drop the price quite a bit with some kind of conventional door. It would also be a lot cheaper to build.
I expect the bulk of a redesign will be a cleaner design that is cheaper to build incorporating lessons learned from the Model 3. To the consumer, things may not look dramatically different, but the car may be lighter with much better range.
If they want to get get money out of existing customers, they could offer a 2170 upgrade pack for existing Model S/X. If it promised vastly increased range, I could see enough people to make it worthwhile shelling out the money for one.
I may not go for it right away. I got lucky and got a primo battery pack. Two years in and 0% degradation with 298 miles at 100%.
Though a 400 mile pack would be tempting. On road trips that would only require one supercharging a day on days with long legs.