No offense to you personally. But anytime I hear people make off-hand remarks like, "Oh, all they have to do is just spend $150 million and that's it!" has clearly never been involved with filmmaking and/or the process of getting a project greenlit. It's easier said than done.
No, I get where you're coming from. I'm not suggesting that it's easy.
Let's put it another way, it's obvious that the economic model for Terminator movies has to be adjusted for what actual returns are. And actual returns are probably somewhere in the $130 to $150 million domestic range, with T2 being the exception. Since you really want your return to be at least equal to your budget domestic, that's basically what the budget for a next Terminator movie has to be to be made.
And, I'd argue that it's certainly possible to make a great looking science fiction action film for that amount of money. Heck, a great deal of cost savings is built right into the initial concept as the Terminators look like normal people most of the time.
I don't want to suggest that it's only a matter of cutting the budget, but if you want to see more Terminator movies, the only circumstance I can see that happening is with more modest budgets. The very best example of this I can think of is Wrath of Khan, a movie that was made for a leaner budget than TMP. Combined with good word of mouth, it formed the economic basis for the rest of the series.
I really think that if there is another movie it has to follow two things.
1. It has to be more modestly budgeted. $200 million Terminator movies aren't a good economic model.
2. It has to move the story forward. Not just in time. Get to JC sending his own father back in time to his death and then beyond there. And follow the consequences of that on JC. Get to the part of the story that we don't know.