That seems fair. As an inactive player you keep your points but loose your ranking until you regain activity. Next to the points it usually says "3rd" or whatever but now it says "Unranked". You don't have to have a point or rank penalty coming back in, you just come back into the rank where your points would place you.

There could be participation points to increase everyone's points a bit over time rather than keeping the average stuck at 1,000 for infinity and boss battles where the charactes who win get a lot of points and the boss is a NPC so doesn't loose points for the people controlling him. That could help the more active players get higher then the ones who drop out and then come back later without penalizing their own points (other than their forfeit in the match if they dropped out mid match).