In this thread you can find the historical context: [FOLLOW UP ON ITEM] New canisters will only be accessible through the icp0.io domain. Existing canisters will be accessible both through ic0.app and icp0.io
It would make development way more painful and links may not work as expected. If the canister id is in a query parameter then SPAs break. If it’s in a header then ordinary links would not work
If you want to try the no-subdomain approach try developing a dapp with Safari as your browser (painful, don’t actually do this!). Safari does not support localhost subdomains and it makes everything harder