Well considering the argument is that what is practiced today as Buddhism is a small subset of what was believed/practiced previously, you would in fact expect

Well considering the argument is that what is practiced today as Buddhism is a small subset of what was believed/practiced previously, you would in fact expect all that stuff to exist... And the fact Borobudur was only excavated in the mid 1800s and wasn't reconstructed until recently isn't exactly evidence that it was still considered an important part of the religion before then.