It’s true that there is plenty of vegetation to go around in Earth’s modern tropical regions, but what’s less clear is whether sauropods would recognize much of it as food. And considering the massive intake of nutrients needed to sustain an animal of that size, let alone an entire population, they might just starve to death, surrounded by food.
Our current atmosphere has an average oxygen level of 21 percent, amounting to a dropoff of almost half since that high mark a few hundred million years ago. For comparison, it’s about the same difference as the amount of oxygen available at Everest base camp compared to the amount of oxygen in the air at sea level.
All of which is to say, your roaming theme park sauropods might be able to breathe today’s air, but it won’t be pleasant, and it might not be sustainable for long periods of time. The life of a modern-day dinosaur could be one of cramped quarters, even if it was allowed to roam freely over an entire island. Large dinosaurs would likely struggle to find food, and they would be consistently short of breath.