Earlier quoted context omitted.
Aside from the absolute need to make sure you sign out of iMessage on your iPhone before you get rid of the device (so your friends still using iMessage will still be able to reach you using SMS), what is it about the Apple ecosystem that would lock you in that wouldn't be solved by using the iOS version of Google Drive to back up the iPhone before you sell it?
It's not the actual selling of the device the problem, it's the way you are locked in the walled garden. Let's say, you don't like the next iPhone. You want something different. Well, tough shit, you will need to find a new way to store all your photos and files since you won't be able to use those either on any other device. Same goes for anything else you might have in your iCloud. Now, I'm not saying that you can'…
All the user would need to do is sign into their new Android device with the same Google account they used to backup the iPhone.
Copying user files from iCloud Drive to Google drive (or any other cloud storage option) is likewise extremely simple.
I'm not aware of any lock in that isn't related to apps that aren't available on both platforms.