Seem to me like Europe in terms of experimenting with rewinding is much far ahead of us and from what I've heard. So why not experiment with the idea of Pleistocene rewilding? I mean fencing off a large area of western land and reintroducing species that went extinct there or ecological surrogates. Simply one can't object to the idea of experimentation and it would still be a long process. If animals show any negative affects with coexisting with the isolated native megafauna, they could easily be taken out, large animals that are readily tracked can be easily removed.
So why not experiment?