It's literally just downloading the client from https://piston-data.mojang.com/v1/objects/4a2fac7504182a97dc... where mojang makes it publicly available.
It's an older version so it's not linked on their website anymore, but you can find the same download link on the wiki: https://minecraft.wiki/w/Java_Edition_1.2.5
What makes you think they will have "lawyers in their future"?
The fact that the file comes from Microsoft's server is one of those "technicalities" that you've heard about some people sometimes, but not usually, getting off on.
Microsoft will probably argue that the login prompt is a technological protection measure, which makes it blanket illegal to work around, due to the over-broad DMCA. That technicality is about as likely to work against you, as the "it's from their server so I didn't distribute it" technicality is to work for you...
Mandatory disclaimer: I am not a lawyer and this is not legal advice.
Right now I am playing Enigmatica 2: Expert, which is a mod pack for Minecraft 1.12
Today I need to teleport to my spaceship which is in orbit around a distant planet and then fly it back to our solar system and visit Mars, as I have neglected to obtain materials from Mars which I need to build this end game item I'm working on.
It's still recognisably Minecraft even though I'm working with a 9x9 crafting grid. It has internal consistency, dragons hoard rare metals, Plutonium is a rare metal, therefore make sure to bring hazmat handling equipment if you're about to plunder a dragon's hoard.
Also you should probably advice mojang to put some authentication on their download URLs, which have been publicly documented for more than a decade by now.
There is no login prompt. This project is over a year old. You are just misinterpreting the situation. This is like threatening to sue Mozilla because you can use Firefox to click on a download button, or Microsoft because double clicking on an exe on Windows runs the application.