If a plugin is truly abandoned, then forking is the only way to go (I just did this for a plugin in the repository, there was no response from the previous author).

Forking makes more sense in general I think, owners that don't have the time to maintain their plugins seldom have the time it takes to develop a relationship with someone new to get to the point of trusting them with their hard work.

The problem of the repository going stale should be looked at separately, if a plugin really hasn't been touched in 2 years for even minor updates (like testing with new version of WP and updating the readme) then perhaps it makes more sense to move them to an "Archive" repository that would be searched separately from the primary one.

Movement between each repository would be automatic and would be an incentive to make sure you update your plugin at least once in a while.