Airhammy posted... I think it generally comes down to 'it's an unneeded film' aka cash grab. I'll explain.
Take Toy Story 4 for example. TS3 ended the trilogy perfectly with a heart-felt ending that made you want to cry. Sure there were a couple of shorts on TV afterward, but did we need another film? I didn't mind TS4, but it was unnecessary.
Friday the 13th. The first movie establishes the lore. The sequel introduces Jason as the threat. The third builds it up, and the fourth was meant as the final. It's literally in the title. But they saw the popularity in the films, and another 8 have been made since.
Halloween concluded the Michael Myers chapter after two films, and from the third on was meant to be a spooky anthology series, but fans asked where Michael was. So they ditched the anthology idea and brought Michael back for a follow-up trilogy in 4-6 which messed up the continuity. and now we have multiple continuity splits with even more films after that.
It's not that Terminator 3 is bad, it's that many fans were satisfied with the conclusion of Terminator 2, and feel the story didn't need to continue. As said in the films, "Judgment Day is inevitable".
I felt that Toy Story 4 ended the series in a better way than 3 ever did. I never understood peoples emotional response to 3.
Overall, 3 is the better film though.