I managed to figure this one out. It appears that XCode 15 needs to have the iOS 17 simulator runtime installed, or it won't show you any of the previous ones you might have installed.
To fix the issue I did the following:
I downloaded the iOS 17 simulator dmg file from the developer portal (https://developer.apple.com/download/all/)
Installed it using xcode simctl as per the last line of instructions here: https://developer.apple.com/documentation/xcode/installing-additional-simulator-runtimes. For example xcrun simctl runtime add "~/Downloads/watchOS 9 beta Simulator Runtime.dmg"
Once I did this I re-opened Xcode 15 and it spent a minute or two verifying the newly installed runtime. And now "voila" I have visible runtime destinations including iOS 17 and the previously installed 16.4.
Seems silly that Xcode 15 needs the iOS 17 runtime in order to work though. I'd call that a bug.
drfrankencopter
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Installing the iOS 17 simulator runtime got things working on my system