Can the old Minecraft launcher harm the PC?

I once wanted to play MInecraft's Alpha, but you need the old MC Launcher for that. If I then backuppe the.minecraft folder, there's still the question whether the old Lancher from Minecraft and the alpha version of Regedit from Win 10 and other things could harm?

No, it can't hurt, but as far as I know you can play it normally in the new louncher, just download the appropriate version

No
You don't need the launcher to play the alpha

No. Unless you're downloading the launcher from a third-party source.

But there's only 1.0 in the new launcher

I found a tutorial on YT where a Mediafier link is linked to a .jar file in the video description

No, in the new one you can only play 1.0 as the oldest

Click on Show historical versions

Show historical versions then it is also very easy with the new launcher.