2Catch
You're right! We just need to introduce a penalty for missing a game. Then the problem solves itself! For example, if you miss a game, you get -10% (you can adjust the number!).

Okay :) In my example, let's say I haven't participated in any tournaments at all. We have:

(0 + 1000)/2 = 500
(500 + 1000)/2 = 750
(750 + 1000)/2 = 875

Let's change the conditions for the leader a bit. He also has 1000 points, but if he doesn't play, his rating will be 900 points. If he plays, but doesn't have enough time and the result is only average, at 700 points:

(1000 + 700)/2 = 850

It's still more advantageous to skip than to play below his champion level. And again, the last three tournaments have much more weight than the previous 33.