All of these launchers contain modded content for Minecraft, which is a proprietary game owned by Mojang. They circumvent the copyright and license by never actually distributing any part of the game themselves, but having the launcher (which is on the players computer) download the original files from the official source.
Could Minecraft modify their license to explicitly deny the use of their game through modded launchers such as these?
Practically, there is no maintainable way to enforce this. It's impossible to filter requests coming from a specific client unless the client reveals something which that would prompt the server to do so.
Legally however, is this enforceable? Can a company such as Mojang change their license and immediately make all this existing software illegal? What if these launchers published the open sourced code for their applications and a regular player compiles it and uses it to download Minecraft. Would that violate the license?
Technically all these launchers are doing is acting as an extremely specialized web browser for the user to download the game from its official source. I feel like it would be the equivalent of Mojang saying "You're not allowed to download our game with Google Chrome from now on!", and also making no changes to their website to prevent that.