It takes two years for the hardware to catch up.
Why is this such a big drama? id Software did this on every release on the doom engine for 20 years. A game that pushes the limits often needs some hardware lifecycles to come into full awesomeness. Better this than dumb it down and have us yelling for better graphics in 6 months. Better hardware is inevitable, most players over the lifetime of this product are going to upgrade their hardware and be able to run on ultra eventually. And everytime they buy new hardware they will get a pleasant surprise that PC runs even better.
Just be patient for now. The game runs fine with small parks. Would it be better if they restricted our park dimensions? I think not. I'd rather not have that constraint and put up with constraining myself to a size appropriate for my hardware. Upgrade later.
Thanks for making sense. People who say the game is badly optimized don't know what they are talking about.