It is not always about speed. There are still plenty of places in the world where people are on limited data plans and to them using delta rpms makes a lot of sense. They can work with slow speeds but not with high data expenses. So i feel turning it on by default and having a setting to turn it off is still a sane choice. Just my 2 cents.
Did you read the other replies? It doesn't save much and some have said it causes even more downloading. And it's even worse if there's a long time between updates, which is more likely for someone in that situation.
The problem is with ‘discarding generated deltarpm data’ every day; please see earlier discussions where Kevin Fenzi et al. explains the **implementation/policy** issues, for instance at [1].
Deltarpm, if implemented to meet its original purpose, will be very useful to a lot of people. It might even be useful to all Fedora users in general as it should reduce network transfer overall. Deltarpm did reduce a lot of update download size for many years since 2007; it would be fantastic to fix the implementation shortcomings and maybe make it useful in other use cases (Edge/IoT) and prepare it for a better future?
[1] https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/...