I'm usually inactive for 5-6 months because I go away on frequent holidays each winter (but not this year - I wonder why), which makes buying a subscription for the 1-2 weeks between each trip unattractive. That's enough for my rating to drop by about 3 (or maybe even a bit more) if I recall correctly, so I suspect the current rate at which ratings decay is adequate to clear inactive players from the leaderboard within a reasonable period. It would, however, be interesting to be able to see stats for just active players, though no doubt there would be disagreement as to what constitutes an active player.