The Paris "Agreement" is a joke. Essentially, every country gets to pick its own benchmarks. So, surprise, surprise, the benchmarks are extremely close to what countries were already projected to hit, and well within the range of possibility without any government interference at all. The developing world, of course, gets to keep burning fossil fuels like crazy (as they should--climate change is a fair price to pay for bringing billions of people out of abject poverty).
The real purpose of the accord is to allow world leaders to score political points by showing that they care about climate change, without actually doing anything. Here's a useful rule of thumb: If every country in the world except for 2 have joined an agreement, then it's not a real agreement; it's theater. Nicaragua, incidentally, saw through the farce and wanted none of it. It's actually surprising that more climate change activists aren't opposed to the agreement. It's like they too care more about virtue signaling than about doing anything meaningful
So because the agreement was such a joke, there aren't any real (non-political) costs of leaving. Inside the agreement or outside it, every country is going to do what it was going to do anyway. The U.S. isn't getting "left behind" in this case because nothing meaningful is actually happening. (As an aside, when people talk about the U.S. being "left behind," try to unpack what they're saying. It's a cliche from negotiation theory, but it isn't applicable in every case where someone chooses not to participate in a negotiation. The key question is whether an agreement will be reached without our input that will have a real effect on us. The Paris Accord doesn't qualify.)
Trump's justifications for leaving the accord are, naturally, a jumbled and inconsistent mess. He says that this was a bad deal for the U.S. and that it would hurt our economy. But he also, earlier, said the deal was pointless. He was right the first time. (Here's another hint: When major industries are all in favor of a climate change agreement, you can bet that the agreement won't have any practical effect and that the industries just want to look good.) The actual point of leaving is symbolic. It plays well to the base, and it drives liberals absolutely insane, which also plays well to the base.
Finally, it's important to realize that the U.S. was never really "in" the agreement. This was not a senate-ratified treaty; it was just an "executive agreement." The legal status of such agreements is still somewhat murky, but one thing is crystal clear: The president cannot go beyond his constitutional powers in implementing such an agreement, so he needs Congress to enact legislation. Which obviously makes sense. It would be a crazy system if the president could just "agree" with a foreign power to do something domestically, and thereby bypass the entire constitution.