An[] alternative way for Wikipedia's administration to lower exit costs would be a mechanism by which prospective editors who disagree with Wikipedia's policies and editorial decisions can publish on Wikipedia's servers their own version of (some) Wikipedia entries and by which users could indicate a preference for getting the alternative version instead of the Wikipedia version when an alternative version is available.

A mechanism for doing this exists already, although it is not endorsed by Wikipedia's administrators. See wikinfo.org, which encourages importing existing Wikipedia articles and rewriting them to suit various "sympathetic points of view" (SPOV).

Citizendium does the same, but focuses on stricter expert oversight and editorial review rather than a different neutrality policy.

The recent FSF transition from the GNU FDL to CC-by-sa for large public wikis had the side effect of considerably enlarging the corpus of Wikipedia-compatible content, which also indirectly lowers exit costs from wikipedia.org.

ETA replying to PhilGoetz: This comment might have been modded down because it doesn't directly address the issue of publishing alternate content "on Wikipedia's servers". As an Internet landlord, the Wikimedia Foundation would not look favorably to such proposals. However, since large public wikis tend to share the same Wikipedia-derived naming conventions, this does not affect switching costs in practice. In fact, it is good for resilience if content versions are hosted by multiple competing groups.