I haven't seen anyone completely remove and replace the entire rear OEM rack, nor have I seen too many other options for rack extenders outside of the Polaris one. Unless you're going to custom make one yourself, you may be out of luck.
You're definitely not the first one to express concern with durability of the plastic racks. That being said, I also haven't seen or read about too many actually problems with them either. I don't think they're cracking on people or anything. It's likely you'll never have a problem. Good luck.
Just a thought, but MAYBE it would be possible to buy an OEM rack for a 570. They are redone with a metal frame, I believe. Idk if the mounting points would be the same though. Could be worth looking into.