SUSE and Secure Boot: The Details (SUSE Blog)

That still requires a fairly targeted attack -- you have to MITM the installation instructions and the ISO download, such that they access your text and your bootloader instead of the real one, and you have to do this while they're attempting to download and install a new OS. (And if you can do this MITM, you can as easily trojan the text instructions to say "go turn Secure Boot off", which is something lots of legitimate small distros will say.)

In other words, it's perfectly fine to accept that this attack exists. It still helps people from being infected with boot-sector viruses when they're not reinstalling their OS, which is the goal of this process. In general, people do not switch operating systems all the time; at most they tend to do so once or twice. Closing an existing vulnerability all the time except for during OS install is still a benefit.