You’re mistaken, you don’t only connect to your iCloud from iPhones. You connect from any compatible client; and the effort that has gone in to the Mail client

You’re mistaken, you don’t only connect to your iCloud from iPhones. You connect from any compatible client; and the effort that has gone in to the Mail client for iOS means it’s a decent enough mail client for non-iCloud mail accounts too. Apples closed ecosystem is mostly its developer tooling and iMessage.