I can't find anything supporting what these folks are telling you; however, I have experienced all of it, with some twists.
Having lost my Silver medallion, I flew from ATL to HNL and boarding pass still reflected status. On return boarding passes, the status did not print and was not able to board, etc. etc.
In addition, booking tickets in advance, more than likely there will be schedule changes/equipment changes etc, and you can potentially lose your "preferred" seating assignments. I did have success by having copies of the previous seat assignments and was ultimately given my original requested seats.
MSUs are useable -but it was difficult for me even with status. Now that everyone has unlimited upgrades, I would think that an upgrade for someone without status would be a stab in the dark but they are useable at the airport until they expire.