So I figured this might be a good topic for me to chime in with. I have a couple fairly decent sized channels that i have grown or helped grow (GizmoSlip @ 140k subs & BeyondSlowMotion @ 80k subs). I also do a lot of backend work producing content with some fairly massive YouTubers ranging from FinalCutKing to Steve-O. I get a pretty strong view into the backend of what happens on a lot of these shows and personalities. There are many things that drive content into a wall, but contrary to what seems to be the popular opinion here, most of that isn't the greed getting the better of them.
Of course, there is a million reasons why a channels quality will suffer, but most of the time it has a lot more to do with that people change over time. Even core life interests change a lot. At the same time, they have these really successful channels that took them years of hardwork to build and often are the leverage they needed to start working on their new endeavors. If they leave those channels alone? They lose a huge chunk of what they worked to build, so they keep on even when their interests lie elsewhere.
Re-inventing a channel is also stupendously hard. With GizmoSlip, we got really popular off of droptests for phones. Hell we standardized how drop tests even happen online. Pretty much every major channel from SquareTrade to AndroidAuthority had mimiced aspects of our show. However episodes were hella expensive and we would have to wait for their to be devices worth dropping. So we also started dabbling in tech news. However our audience who was trained to just expect droptests from us, got harsh with us about the news s**t. We would not only get way less views, but our audience would tell us that they didn't want any of this news s**t and to stick with just the drop tests. It was disenheartening to be sure. I eventually left GizmoSlip to my friend Brandon who i started it with to just focus on BeyondSlowMotion where i was more passionate about the porjects in the first place.
Basically I had to leave a much more successful channel for my less successful channel (at the time i was only 18k subs) in order to find the drive to continue creating. This is part of why it is so important to be careful how you establish your online identity so that you can be more flexible with what and how you create. It is easier than you might expect to get caught in a rutt both creatively and financially and to have to be dependent on something you don't have any passion for. I'll back a lit of what someguydude was saying about this stuff. There is a lot of truth there.
That said, I am loving life with BeyondSlowMotion now and consequently make more money than I ever had in the past with GizmoSlip and or even in the traditional work force. It is important to be upfront early on about what you actually care about so you don't get caught too heavily in following rabbit trails to nowhere.