We have no idea what the bounds of our inventory is. We do know however that those of us with a ton of pins (over 1,000) begin to have inventory and trade issues. They have worked on the issue on their end as much as they can as far as I can tell.
Doing the Math on this they figured out that if they continued to put out 5 pins monthly, virtually all players would eventually end up bogged down. Most of us Pin Collectors buy 10 - 20 of each pin VMK has been open roughly 150 weeks so if you extrapalate that it's 1500 - 3000 pins just for weekly pin sets. Obviously they stopped a while back but I do infact have 2000 + pins and I can now do trades but after a trade or two I have to restart.
They simply could not sustain the current pace and maintain integrity in the system, This is my opinion based on the facts above and that they stopped making new series when they corrected the issue with trading.