Both the Fuji 50S and R are based on technology originally released nearly 3 years ago.
Having looked at what Sony is doing with sensor products, there doesn't seem to be any intermediate product between 50 and 100Mp for the mini-MF sensor size, so the next logical step for the smaller bodies is up to 100Mp. The 61Mp FF chips announced for the Sony and Nikon FF cameras have the same pixel pitch as the GFX 100 sensor, so it seems that the Sony chip fab is going to be turning out a lot of BSI chips at that pixel size.
When the A7R IV and Nikon Z 61Mp cameras are out, these cameras lose their resolution edge in the market place and pixels do still sell cameras.
A bit of 'kremlinology', where next for the Fuji GFX range and particularly the smaller cameras? I'd guess for some kind of revamp with the 100Mp chip after the initial sales of the GFX 100 camera has soaked up the early adopters and the fab has come up to full speed turning out wafers.
The Sony consumer product map is here :
https://www.sony-semicon.co.jp/products_en/IS/sensor2/products/index.html While this does not include all the products that are rumoured, and I guess specific products are made to order for eg: Nikon that won't appear here, I assume that most products will use the same fabrication plants and so there are limits to the technologies available.
ie: Sony builds out its 3.76 wafer plant then generates a bunch of standard products for these, with enterprise customers doing specific deals for custom outputs from that level of plant.
This would mean that future chips would most likely come from the currently available processes indicated on the product map, even if they might have some custom toppings, the base sensor technology has to be shared across a larger set of products to be viable.
Its a wild guess on my part but I'm wondering if the 2.6 micro Stacked BSI process might be the next one to make its way to FF and MF for the next generation. That would provide a big improvement in noise and a pixel bump. An FF sensor at this pitch is 96Mp and a mini-MF would be 150+ with the Full MF chips getting up to 230Mp!
I'm ready and waiting for someone to throw a bucket of water over me on that. My mouth waters at the prospect of a Fuji 150R sometime in the future.
Less wildly the likelihood is that the R and S will stay at 50Mp and get some kind of internal refresh, maybe like the Hasselblad X1D II which is staying with the 50Mp sensor. Maybe the S and R won't see the 100Mp sensor at least until more of the big camera features can be miniaturised, such as the IBIS to make the use of such resolution more useful in a hand held package.