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.