Fatigue may or may not happen at the box office (it all depends on the hype narrative and whether the general consensus is positive or negative) but Im sure from a narrative standpoint if they dont majorly scale back production after 2019 to at minimum one film every three years theyll drive the franchise into the dirt.
Granted, Im not a huge fan of the new films (though I still kinda enjoy TFA), but it seems like most people think TFA was pretty good but not better than the OT (maybe an 8/10), and Rogue One was flawed but still enjoyable (7/10). If these are the two films theyre starting out with, how does Disney expect to maintain a consistent level of quality with these fixed release dates?
How many filmmakers have really great ideas for a new film, and that the studio is actually willing to hire and not interfere with? How many of those filmmakers actually want to make a Star Wars film, with all of the restrictions and scrutiny and potential career consequences? Im sure there are plenty of journeyman directors that can churn out decent and inoffensive products, but these films should only be made when there is an innovative and narrative reason for it. Turning Star Wars movies into an assembly line will not produce the culturally significant and classic films that the OT is known for. How many people here want just okay Star Wars movies?
Star Wars isnt Marvel, there isnt a plethora of sources and styles you can mix it up with. For most people, Star Wars is the OT, and the further you move away from that the less interested theyll be. I can see the brand being diluted until its like Harry Potter/Fantastic Beasts or the Hobbit, still making a decent 600-800 mil per movie, but is that really what Disney will be satisfied with? I think the MCU films will also start reaching a plateau, with none of them passing $1.1 billion aside from the occasional outlier (like Infinity War part 1). Sure, theres absolutely nothing wrong with making that much money per movie, but I dont think the corporate overlords will see it that way.
Again, all of these thoughts go to the legacy of the franchise, not how much money it continues to make or even the average audience consensus. If Disney continues on this course, Star Wars wont mean the same to kids now and in the future as it did to people who grew up with the OT (and even the prequels). I realize that everything changes, thats just the nature of things, but is it really ideal for it to become just another series, as interchangeable and innocuous as the next? Sure, the commercial aspect of SW has always been shamelessly moneygrubbing, but should films, and heart and soul of the franchise be that way too?