I don't know if this is even relevant now that I finished writing it, but I put work into it so I'm posting it.

I think it kind of depends on how you look at Reddit. If you look at it on face value, then you're absolutely correct, it's a glorified message board and people are free to leave when they get bored. End of story.

If you look at it as a business though, the picture is different. As a business, Reddit needs to make money to continue operating, and it makes money by maintaining and growing site traffic by providing the service of connecting millions of customers (users) together in one place in a fashion that those customers are happy with. These customers have become brand loyal to reddit and reddit has just made some major changes to their brand without doing any alpha or beta testing, and are now seeing the result of that failure to plan ahead and try to cut costs as quickly as possible. The size and popularity of that brand magnifies these effects. If there is a suitable competitor to steal those customers, then reddit will lose revenue and possibly fail over time. If there's not, then people will deal with the changes and this will all blow over. We'll just have to see what happens. By your description of what happened to Digg, it was, essentially, the same thing that is happening here. It's a change of the service/product they're offering.

TLDR; Under the surface, reddit is a business. The customer base as a whole will punish reddit if there's somewhere else to go as a result of these changes. Because of it's size though, there might not be enough competition for people to jump ship. We'll have to see what happens