> Weak players come into the chess.com pool faster than they leave.
By definition, someone entering the pool enters with the average elo (for a closed system). The rate of entry and exit doesn't affect the direction of average (open system) elo. (It does affect the rate of divergence; the faster entry is, the longer it will take for the open average to diverge from the closed average.) What matters to the direction is the elo players have when they leave, not how long they take to leave.
Consider what happens if people enter with 1000 elo at a rate of 10 per day, and leave with 800 elo at a rate of 3 per day:
+-----+---------+-----------+-------------+
| day | players | total elo | average elo |
+-----+---------+-----------+-------------+
| 0 | 0 | 0 | 0.0 |
+-----+---------+-----------+-------------+
| 1 | 10 | 10000 | 1000.0 |
+-----+---------+-----------+-------------+
| 2 | 17 | 17600 | 1035.3 |
+-----+---------+-----------+-------------+
| 5 | 38 | 40400 | 1063.2 |
+-----+---------+-----------+-------------+
| 10 | 73 | 78400 | 1074.0 |
+-----+---------+-----------+-------------+
| 20 | 143 | 154400 | 1079.7 |
+-----+---------+-----------+-------------+
This... is nonsense. That has nothing to do with the average elo.
The situation you describe is mathematically impossible. Please consider that before you repeat the claim.