I don't think Plato was advocating the belief of a world of perfect forms.
He invented it.
Yes, but he didn't claim that it exists in the same sense as the computer monitor you are looking at and the chair you are sitting in exist. He claimed that it exists in the same sense that the numbers
one,
two,
three, etc. exist. I assume you don't deny the existence of the number
one, but you never observe the pure number
one in the physical world. You may observe one apple, or one chair, or one computer monitor, but you don't observe
one as a disembodied abstract entity. To Plato, the perfect form
chair exists in the same sense as the abstract entity
one. Essentially, he's making the distinction between objects, which exist in reality, and concepts, which exist as much as objects do, but in some way which is distinct from the existence of objective reality. However good or bad he was as a philosopher, I don't see why you find this idea so objectionable.