I like the idea. One additional modification to the Staking contract would be to make it so closeAllocation can be called by anyone instead of just any of the delegators.
A milder version of this would be, instead of slashing the indexer, just distribute rewards (if any) to the searcher. Good thing is that is not too harsh on an indexer having issues to close those allocations, however, if the penalty is too mild it might not be enough deterrent, and in some cases I guess rewards can just be too small to compensate the searcher.
You can see here Dune Analytics the number of allocations that are currently over max epochs.