As you have discovered, changing a file’s filename extension under iOS/iPadOS isn’t possible. Drawing direct comparison between full “desktop” Operating Systems and iOS/iPadOS is problematic as they are fundamentally different - in particular in the manner that they are designed to interact with the User.
As you are doubtless aware, the filename extension is used to identify specific the specific file-type - and the Apps that will “open” a file for viewing. Whilst a full desktop OS permits renaming of a file and its extension, this creates opportunity to inappropriately alter a filename extension.
iOS/iPadOS does not provide the facility to alter file extensions. File extensions are just one element of the underlying filesystem that is effectively hidden from the User - the OS being designed to internally manage its filesystem and file associations. When accessing a file, iOS/iPadOS will open the [internally associated] App that is capable of reading the file - either directly or via the system share-sheet.
The Files App provides limited access to internal file storage, iOS/iPadOS does not expose its actual filesystem; this is architecturally “sandboxed” such that individual Apps can only access their own area of the filesystem.
If you wish to change a file-extension, you must do using a different computer. You should be mindful that changing the extension may cause the file to become unreadable by an iOS/iPadOS device - as, the device will attempt to open the file using the App inferred by the extension if a corresponding App is installed.
Apple do invite constructive comments and feedback via their product feedback pages:
https://www.apple.com/feedback