Microsoft is a member of the RIAA. It could and should resign.
Microsoft can lobby for further exceptions to §1201 anti-circumvention.
Microsoft can issue a statement formally protesting RIAA's action.
Microsoft could offer an Amicus brief or other statements in favour of youtube-dl developers.
Whilst we're discussing monopolists, Google could issue a statement that the rotating is not in fact an anti-circumventiion device.