I think you misunderstood my comment. I already acknowledged that a complete system would indeed need different tuning for each map, which may or may not require adding extra back end data collection. My question was, given that the Champ Mastery system has been up and running for a while already, why Riot doesn't temporarily use a bare bones algorithm to distribute points.
As far as I know, Riot hasn't mentioned working on expanding the system to the rest of the maps. All comments have been about adding new rewards or Mastery levels. If the Mastery team can't add the full system to the rest of the maps in a reasonable time frame (whether due to incomplete data collection, being undecided about how to grade gameplay, or something else), why can't they distribute points using a rudimentary algorithm with the data they do have? Use win/loss and length of match to grant Mastery points. Maybe even reduce the final points by 25% or so because it's incomplete. Instead of displaying a grade in the post-match lobby, have a little disclaimer explaining "We haven't finalized the grading algorithm for this map yet, but we wanted to let you players enjoy Champion Mastery in the meantime."
Again, if Riot adds the Mastery system to the rest of the maps soon, then this little delay is understandable. But it won't be understandable if all the alternate maps continue to be excluded from Mastery simply because the "post-match grading system isn't ready."