I guess I was thinking of a site that the of entirety (or nearly all of) the 10/101ths of Reddit users which make up the theoretical strong feelers would move to.

As in order to attract most of them a single site would need to both be able to handle the traffic and have a broad appeal across subjects. Maintaining such a site with that much headroom for users with only their current user base to generate ad revenue I assume is problematic and I would be surprised if any single site currently could handle a mass migration like that (except maybe Tumblr which I have my doubts would appeal to this portion of the Reddit population).

So while a mass exodus could take place people would likely scatter to different sites, which might not make any single other site 'it' enough to attract the remaining masses. Or the exodus takes place over time, which I consider the likely situation, having seen the effective death of many internet communities prior to the beast which is Reddit.

If they leave over time Reddit doesn't need to ask what it should do when the 10 leaves, it needs to ask what it should do when the 10 starts to leave.( my guess would be they need to get them to stop leaving, or get others to step up and take their place as people that feel strongly about the site in its then state, or buy the site they are leaving to).

What was the proper successor to Slashdot? Digg?

I somewhat lost touch during that period. When I came up for air, it seemed to be HN, StackOverflow, and Reddit (~2009 or so).

SA and Fark were both pretty big internet communities back in the prime of Slashdot. In that I remember there were quite a few friendly (and less than friendly) rivalries going between the three.