If we are strictly talking on-boarding, can't argue with that. A simple and secure solution first/default. If they want to leave/branch to other clients they can learn how to extract their keys, or try more secure management solutions.

Once client apps get better I can't imagine there will be as much client swapping as we do today. So it doesn't have to be easy to swap clients. I wouldn't swap so often if each client didn't have major UI issues that cause me to switch between them.

I also have to imagine some (or most) mobile clients will simply setup a nip46 signer in-app in the future anyway so if users want to try other clients they just need their starter app which is currently holding their keys, no nsec extraction necessary.