This is one of the biggest plot wholes throughout the entire LBT franchise. With the exception of the original film, where the Great Valley is portrayed as a metaphor for Heaven, or as the final resting place for the last dinosaurs on Earth, it never made much sense that big dinosaurs like Apatosaurus and Triceratops could find their way in, but Raptors and T-Rexs couldn’t.
I know this theory about the first film is debunked by the very existence of the sequels and the TV series, but since the latter weren’t planned when the original film was released, you can still attribute this meaning to the artists view/interpretation of the overall story.
In LBT II we got at least an attempt to explain why predators couldn’t get into the Valley, we’ve the implied sense that the grown-ups had closed the only passage way, and blocked it with rocks. In the TV series, in the episode The Hidden Canyon, we, once again discover that the tree sweets in the Hidden Canyon keep sharpteeth away, so maybe the wandering herds are using that passage to get into the Great Valley.
But let’s ignore all of this for a second and just assume that for some reason the Great Valley is just magically protected from sharpteeth. If it were ever to become unsafe, I believe two scenarios could happen. Either the herds just stay there and organize themselves in a way that some dinosaurs would patrol the entrances to try and warn the others or stave off predators, if they ever try or manage to get in. Or, they would just leave, like they did in LBT V and find another safe place to stay.
Our biggest mystery is what’s the mysterious beyond like (I know, it’s redundant). If it’s full of leafs and green valleys like we’ve seen before, then they would probably just migrate to some other place, if it’s dying and leafless, then they would most likely stay in the Great Valley and just ward off sharpteeth, like they did before they arrived in the Great Valley.
As for Ozzy and Strut, scavengers are needed for a stable ecosystem to be maintained. What happens to the corpses of dead dinosaurs? Someone has to eat them, otherwise they would just rot there, which could be a source of disease for the other dinosaurs.
Even Guido is a scavenger, although he found himself in the Great Valley because he sleep-flew there.