Okay. I understand a little better. It seems like a room-type guarantee applies rather than a URG because you had a room, but not the type you booked. Under the room type guarantee, you get $100. 50k MR points > $100, so you are whole from the property’s point of view. I personally wouldn’t hold my breath for any more.
The OP's posts are really unclear, but yes, it appears this was a downgrade, not a walk. It's also not clear what rate was charged for the downgraded room. So it's hard to tell what the appropriate remedy is, and whether it's been provided.
I agree 50k points is not bad for a downgrade, particularly if there was a rate adjustment for the downgraded room. Also, the 50k points will be charged to the property by Marriott, so they do wind up paying for their service fail.