Game companies need to understand that they cant have a monopoly on peoples time. Getting bored of a game happens when people play through the content that they release. People left in droves because of the constant nerfs and sandbox changes that put more and more restrictions on how people wanted to play the game. The attempt was to freshen things up but instead what they should've done is let people get bored and take a break for a month or two and come back. Simply put bungie doesn't have deep enough build mechanics and diversity to keep people engaged for a long period of time and they shouldn't be trying.
They lack any real imagination. When difficulty to them means just adding bullet sponge tanky enemy's with different shields, long jump puzzles, and timers and stomp mechanics that knock you across the map. There's a problem.