There is a good question...What are the parks for?
Should they be pristine, untouched living museums? Do we need to get them back to the state they were 500 (pick a number) years ago? If so, we need to spend billions to try and reclaim them. I think there is more important places to spend our tax money. It is probably a good idea to try and limit importing new invasive species to the park, but the effort to return things to somewhere near a baseline shouldn't be that high a priority when you consider the state of how many people exist in our country.