I am not sure that thinking of this is a trilogy is correct. its the 8th , 9th movie.. this is not a short arc like Avangers, its a 40 year arc, that was building and diminished because of the story in 1-2-3, even in still setting records. This one brought the gang back together, and by that i mean families, kids, grandparents and the 50-60 yr olds who loved star wars.
How many series span enough time to have to 3 sets of fans, two of which love their trilogy? its the same reason this went huge because the model didnt know how to handle it.
star wars went down in ep 5 because people who went out of curiosity knew whether they liked it and did or didnt go back for another round. it didnt matter if the movie was good to those who didnt enjoy the taste they got from that story telling.
Ep 8 will be far different, the curiosity factor will be far lower with an established 40 yr franchise. This is why Bond can explode 50 years later, This time around it will purely be about the story and the reviews. It will go huge with the fans out of the gate but if the story is there it will huge again. The one thing that can hurt it is the next movie if it tanks for being bad. but if its good it will fuel the fire even more. Sure it can drop 30% , but its going to be at 500 million after 2 weeks again and it if its good it will be 850-900.