I'm experiencing exactly the same issue. I'm also using an iPhone 17 Pro, and my situation is almost identical. The "Try New Siri" option appears, I can go through the setup process, and the settings change to "Siri (Beta)", but when I actually use Siri, it still launches the old Siri interface instead of the new Apple Intelligence version. I also noticed that opening siri:// through Safari launches the new Siri app, but it gets stuck on "Siri Update in Progress" and says that adding support for Siri hasn't completed. The app also doesn't appear in Spotlight. From my perspective, this looks like the device is already enrolled for the new Siri, but something during the activation, download, or registration process is stuck. Personally, I wouldn't downgrade back to iOS 26 and completely restore my iPhone just to fix this one feature. My iPhone 17 Pro is working perfectly otherwise, and going through a full restore, reinstalling everything, or setting up the phone as a new device is a lot of effort for something that will most likely be fixed in a future iOS 27 update. I know some people have fixed the issue by restoring to iOS 26 and setting up the phone as new, and I'm glad that worked for them. However, unless Siri AI is absolutely essential for your daily use, I personally don't think it's worth going through all that trouble right now. If you're not comfortable with restoring iOS or setting up your iPhone from scratch, I would recommend waiting for the next update and seeing if Apple fixes the issue. It seems more likely to be a software, activation, or registration issue rather than a hardware problem.