Purchasing a new phone is only a small subset of the scenarios where a backup is useful, for example, in the last two years Apple support has requested that I reset my phone and reinstall iOS twice. Having to buy a second phone to temporarily store my data is not a viable alternative.

Or losing your phone.

Or accidentally breaking it.

Or having it stolen.

A backup protects my data in these situations where a transfer utility does not.

Backups disabled by default would be a sensible approach, but is very much being childish not to let users access their own data.

There were mentions of multi-device coming later on Twitter. While it's not the desired backup feature but a mean to duplicate data at least and backup with Android.