It’s pretty small set of people who did a new install and got a plugin installed unbeknownst to them. If it’s easy to make the plugin just disable itself rather than make the build fail, that seems preferable.
I understand correctly, it also means that people who installed the plugin will have their rebuilds fail. I’m less worried about them, though, as if they installed the plugin, they presumably also know how to uninstall it. Even for them, though, some deprecation warning seems more gentle.