Nobody said it was hard. But what's your percentage of people that will "save the code" in a document and remember where it is X months/years from now? But everyone will still have their email for the most part.

Saving the code for some random app? No way. I'm too lazy to save a restoration code for my gmail account...

Alternative you can link an email without it being a necessary condition.

If its opt in 90% of people wont do it and near 100% of users will change their device in 1-4 years. 90% of your users will end up losing access to their account and 50%+ will stop using your service in frustration because of creators design flaw. Sign in with google is already almost as easy you don't have to have an actual account creation flow just a button click and a screen where users browser autofills their password and they click login. 2 clicks and 10 seconds.

I don’t know about android but on iOS, restoring the new device through an iCloud backup will bring back the app data too.

So, the users who switch from iphone to an iphone don’t loose anything if they follow the default upgrade process.

How many services do you use even if occasionally and how much toleration do you have for cleverly different things like magically knowing who you are based on device with some way to link an account after the fact. I have zero. I want things to work like other things work. I don't have time nor desire to figure out the non obvious way things work just because the dev is confused about how special they are.

That’s not the case here. Think of this as each app having a folder in your iCloud disk without being able to see other apps from different developers. As long as you don’t delete the app, the folder comes with you as you move from device to device so the apps installation is not required each time.

Also, lockin...

Sounds great while iCloud is up/you use iOS...prepare for the shitestorm when its not or the user decides to leave Apple... 'Easy' and 'Convenient' won't be the terms anyone is using.

That indeed sounds great for people using ios but 90% of users aren't

I'm pretty confident you could change those numbers with a few reminders to link your email/whatever for a backup. My WA account is extremely aggressive about setting backups, so clearly it works (but you don't need to be that aggressive. You can still respect users and remind them of things.)

I'm pretty sure that if you learn the basics of stitching you will be able to close up the wound after you shoot yourself in the foot.

Give them a popup like "link your email to keep your data even after you switch devices"! Don't give them a cancel option though. Give them a "I want to lose my data" option. That will make them think.

Yep and this could be enhanced further by provide a code retrieval service by email, plus facilities to change the code.