Yes but it's not a "closed ended option" when you have countless millions of years ahead of you, and your progeny, in which to live. They would literally have more time to figure out options for the human race than the human race has even existed for currently. I know that's a brain twister but think about it, humans haven't been around for that long at all. Human civilization has only been around for approximately 10,000 years. Homo homo sapiens have only been around for approximately 100,000 years. (Don't ask me why it's "homo homo sapiens" with two homos, that's just how they say it in history class now. Seriously.) Compared to the dino period this is nothing. It's barely a speck of time. The meteor wouldn't even be on anyone's radar if they went back far enough.
So you see, if they went back to a point where they had millions of years stretching ahead of them (in which they knew the relative geological history) they would be better off than we are now since we don't know what the hell awaits us tomorrow.
And as for why they didn't use some other wormhole plot device, who knows? Perhaps they have and we just don't know it yet. Or, it's just how they wrote the story and we have to accept it. If I can accept space vampires I guess I can accept time travel to a dino world. Admittedly the time travel aspect can get a bit hinky, unless it's done as a Michael Crichton jump-to-an-alt-universe time travel gimmick, then I guess it makes sense. (Crichton's "Timeline" while a bit cheesy and melodramatic as page turners go was also good on the science of time travel theories. At the same time he was also flawed in that same area. But it makes for good debate and a good basis for a fictional story.)
The trailer-or web site-says they go back 85 mill yrs the impact occurred 65 mill yrs ago (http://www.universetoday.com/35116/chicxulub-crater) . So they have 20 mill yrs to "party on"
