It doesn't help the franchise that ownership of the property has changed hands so many times either. The studio has some say, but the company owning it has the bigger say and everyone's going to have different opinions on how to save the series.
Salvation was hurt most by not being the unique future world Cameron created and being like any post-apocalyptic world, and also by rewriting a lot of established canon about the future, yes they get the excuse of things changing by a new judgement day timeline, but still, including T-600s and making them the size of Andre the giant and not looking remotely human didn't sit well with people, Marcus's advanced technology, humanizing Skynet.
If someone could just realize the notion of a future war movie is not what went wrong but the delivery, we'd be fine. Even Genisys reinvented the future war despite the movie trying to ignore T3 and T4 and almost say, this is Cameron's future wherein the original T-800 is sent back. They went to such great lengths to mirror scenes from '84 with Arnold but came up with all new costumes and looks for the future.
i think thats ultimately the problem
every writer and director wants to do their own thing. everyone wants to add their own spin, so unless cameron himself was on board (or a director working under him) that future war movie will never happen