Does Etcher replace rEFInd?

You can do a clean install of Ubuntu on a Mac without needing to install rEFInd.

The procedure for installing depends on the model Mac and the version of Ubuntu.

The rEFInd boot manager can be useful in the following situations.

• You want a menu with operating system choices to appear each time the computer boots or reboots.
• You want to select the default operating system to boot from the currently executing operation system.

rEFInd is a boot manager. This means rEFInd does not actually boot an operating system. rEFInd just instructs the firmware which operating system to boot.

Since you wish to only install Ubuntu on your Mac, then there is no reason to install the rEFInd boot manager.