I think the problem is, fans of the franchise have, for the most part, not enjoyed Rise, Salvation or Genisys. So it's a case of fool me once, fool me twice. Also, the younger generation don't have the same connection to the franchise or the same love as they more than likely grew up with Rise, Salvation and Genisys. Also, apart from Salvation, all the movies are the same basic plot: sentient machines send a killer robot back in time to kill an important leader. Humans send back a protector. Fight! Kind of repetitive, even when you have a movie like Dark Fate that has some fantastic action.
I just don't think all fans think like that. A nice portion have a connection, some don't know, some don't care, and some aren't into it because it's not popular for their age and era.
The Terminator franchise has always been about what it means to be human and what makes us different than machines. It's a question that goes back to early human myths, which begs the question on how humans are different from animals. If you look at it, each franchise other than The Terminator, the T-800 learns to become more and more human
Then the machines arrived on your front door. Dark Fate and its predecessors in the franchise, revolve around artificial intelligence deeming that humans pose a major treat to the future, and they are far from alone. Authors have written about dangerous, intelligent machines at least as far back as the 1870s. But whether you're talking about Samuel Butler’s 1872 novel Erewhon, where he talks about machines that developed consciousness, or Dark Fate, the threat posed by AI isn’t just physical, but metaphysical. The concept of a computer possessing intelligence that rivals or surpasses our own special way of thinking that we are the most intelligent life in the universe. Artificial intelligence and machines have made us ask the question again “What makes humans special?”
Instead, stories like Dark Fate emphasize physiological differences between human and machine. Take Grace for example, she's a modified human that has the conscious to decide between good or bad versus the Rev 9 that only follow orders based on it's design and programming. Can you have a machine that develops it's own way of thinking from lack of instruction and living among humans? Humans face life and death where the latter is permanent versus machines that can be upgraded, repaired, or rebuilt.