It is theoretically possible to make the on-wiki TallyPage work with encrypted elections, but it involves manually entering the decryption key into the DB and tends to time out for anything more than small elections. Since election tallying is mostly performed via command-line scripts, we should remove this broken feature.
TallyPage is useful for unencrypted test/example elections, so it's worth keeping for those limited cases.
I think that is not a good solution; we should instead change the code such that tallying via web interface would be possible. It'll take a lot of work but that is the "solution"; removing the ability to tally encrypted elections would be a workaround, and fells like sweeping the problem under the rug.
Even if we don't allow web-based tallies, at least we can make it such that the tally is done in command line but the result is cached in the DB, and later when someone uses the web-based interface they will be shown the cached results.