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I would imagine because it will break lot of scripts that use yum tooling. Just changing binary name could be painful.

Even on Fedora yum is still available

No, not really. You can install yum, but by default, any yum commands are translated to their dnf counterpart.

(Many don't need translation other than `alias yum dnf` sort of thing)

On my fedora 29 box:

[root@laptop ~]# which yum

/usr/bin/yum

[root@laptop ~]# ls -l /usr/bin/yum

lrwxrwxrwx. 1 root root 5 Oct 15 12:47 /usr/bin/yum -> dnf-3

Pretty sure "yum" in rhel8 will actually be dnf as well, but keeping the name for backwards compatability