I wrote a whole thing on this, but tldr; I switched from Gmail to iCloud about a year ago. Gmail is better at filters, search, web UI, and spam filtering. iCloud is better at native UI, offline support, and ecosystem integration.
Overall I’m happy with it but only because I’m willing to pay the costs for the benefits, which I think is pretty rare.
There’s a survival game called Grounded that’s basically this but even smaller. And a coop story action/adventure called It Takes Two that does the same.