I believe you nailed it with the capacity issues. They are removing landscaping to allow for better throughput of guests simply because when you move in and out on average 33,000 guests per day (probably over 80,000 on holidays) you need much large walkways. Heck, they even added a semi permanent backstage walkway this year because of this problem.
I don't think the other parks are unsuccessful at drawing guests. They are in the same boat as the MK when it comes to being overcrowded. WDW as a whole is far too crowded on many days of the year. Maybe the rated capacity is not reached, but the parks not enjoyable or safe in my book. I cannot imagine if there was ever a true emergency in one of the parks on New Years eve when they are at or near capacity and everyone had to be urgently evacuated. It simply would not happen and many would be injured.