For me, writing meant a world of infinite possibilities, freedom from all constraints of reality and creating something that only you could create.
Why should the fact that people run into ideas already thought of in the past mean that there is not an infinite number of possibilities? I'd think the more basic an idea or creation, the easier it is to replicate.
There has been philosophers, scientists, economists and mathematicians in history who independently discovered similar ideas but that never put an end to their infatuation for their fields. Why should it affect your writing if it's your writing which captivates your passion?
And I think considering only the individual things you create in isolation is taking a narrow view of the issue. I think it matters more the nature of the whole body of your creations. The full list of things you've created in your lifetime is a better indication of a creator's uniqueness. It isn't merely the fact that Thomas Edison created the light bulb which makes him one of the greatest inventors in history - others are said to have created the light bulb independently; it is the fact that he has about a thousand other inventions to his name and the fact that he was self-made man - having only received several months of schooling in his early life. But do you think Joseph Swan (another inventor of the light bulb) would have given up inventing things had he discovered earlier that Thomas Edison would be better known for his own independent invention of the same item? Not if he dearly loved and enjoyed inventing things.
Assuming there is such a thing as 'limits to our imagination,' what does it mean to step outside of these limits? Can we comprehend the outside of these limits, and the very limits itself, at all? Can we know what these limits are? If human imagination cannot grasp these limits or the things beyond these limits, can we come to understand them? Are they within the capacity of human understanding? If not, then isn't it redundant to worry about the content of what is beyond these limits?
And would you agree that there exist no other Tailspin in the world, that your personality and character is unique and there is noone like you? If so, then surely you can agree that there exists something which only you can create - that there is something you can express that is unique to you. And if there indeed does exists someone who can only create the very same things you create, then ought that not draw fascination and awe of human nature, rather than this intense worry and turmoil you're currently experiencing?
My father's theory took all that away from me and such sucked all the happiness out of my life.
I wonder whether you've allowed it to take it all away from you.
To me, the theory says that everything I could ever come up with is simply part of a pre-existing set of possibilities and eventually no one will be able to write anything without it being an exact copy of something else and everything I do write will bring into little closer to that time.
I don't believe you've provided a full proof of your theory, or even a convincing argument to support its truth and validity. Even you have doubts in your mind, which is why you're going around the internet posing questions about its validity. Your love and enjoyment for unleashing your creativity is threaten by the mere thought of the theory being possibly true. Why?
If humanity runs out of new ideas, what will it mean for human civilisation? Will we then, at which point, have no more problems to solve about the world and human existence?
I think when you speak of 'pre-existing set of possibilities,' you mean almost the same thing as scientists when they speak of scientific knowledge as being 'discovered.' This seems to me as bordering on a discussion about free will and determinism.