Rpm is a stat that is designed to measure players average contribution to the game. Its not a perfect measurement by any means but it has been pretty consistent on many things. It usually locates bad defenders pretty well, it quite consistently rates players on top that probably really has the biggest impact and it often demonstrates the fact that rookies dont usually contribute to winning. Disregarding the whole metric doesnt seem justified to me. If player is a huge asset and a force on the floor, usually that metric recognizes those players, or at least it wont rank that player last in his position. If a player is a rookie and he has a bad rpm, its just normal and predictable since even if a rookie is a skilled player, because of his inexperience he most likely wont contribute to winning.
Again, same with Fox last year. You could see he was a skilled player but still he wasnt ready/able to contribute to winning. When it comes to Bagley, its reasonable to assume that he contributes to winning more now than he did at the start of the season and if that is the case, he will most likely rise from the last spot of pf's RPM.
Also you shouldnt go soley on this one stat, no. But if the metric implies that Bagley is last among pf's, then its reasonable to assume that since he is a rookie, he just isnt conributing to winning yet.