Where do you think that new kind of fish would come from if not from finding and adapting exicting genetic materials? We don't even have proper understanding of how our body works and we have been looking at it for a long time.
We are naive to think we can invent ourselves out of a vacuum. In all likelihood the current progress of humanity is leading us to a local minimum.
And when we do, it will be nowhere near existing biodiversity for a long-long-long while (if ever).
And we are also nowhere near doing that at scale from scratch. We need existing living breathing species to do anything.
And even then it's a problem when "new kind of fish" ends up destroying a different kind of fish [1] etc.
I won't even go into how bees are needed for so much more than just crops.
[1] https://www.scientificamerican.com/article/farmed-and-danger...