Earlier quoted context omitted.
As Digg found out many years ago. There is nothing stopping Reddit users from leaving. The question is which straw breaks the camel's back... and also lack of viable alternatives, a la Reddit to Digg.
Apart from the fact there is no decent reddit alternative. That is after reddit offered their entire product free with the source code and data (and the data is still offered for free...) If there was going to be an alternative, it would be here by now. People just don't care that much, contrary to what you may believe.
But a code base is not an alternative, an alternative is an actual community with a similar focus. You could potentially build it on Reddit's code base if you wanted... but you have to build that somehow. Network effects prevent that from forming and from people leaving existing communities.