It was already the default installer in Ubuntu 18.04. So this is the second LTS release where it is the default, and this thread was created to gather input from users about what features it would need to gain in order to completely obsolete the non-default d-i-based installer. I’m sorry this didn’t come to your attention until after the 20.04 LTS release has happened, but it’s quite difficult to reach all Ubuntu users who might be affected by deprecation of a component. This is effectively why the d-i images have been kept in place as legacy images for 20.04.0 LTS, so that users have a solution for use cases that were not captured for the 20.04 release of the live installer. It is also why the path of these images has been changed, so that users know that there are changes happening and can plan for the future.
Bug #1429030 “netboot mini.iso doesn't support UEFI boot” : Bugs : debian-installer package : Ubuntu shows that the mini.iso does not work on any system running in UEFI mode instead of in BIOS mode. That means it is only usable on an ever-diminishing percentage of older server hardware.
I understand that you find the mini.iso better suited to your needs. But I don’t think it helps for you to assert that it meets the needs of “most of [Ubuntu’s] users”.