The presumption is a failure to appoint electors in PA would be equaled in WI, MI, GA and AZ. I think even that would not suffice. But it seems impossible the contingent election in the House only takes place when the count is exactly equal.
It's not when it's exactly equal, it's when nobody reaches a majority of the votes cast.
The only way it can happen with two major parties is an equal vote of 269-269, but historically it happened with multiple party candidates receiving votes.
If Perot or Nadar had received votes, it might have been enough to keep the leader from getting the majority.
If more states move to proportional elections like Maine and Nebraska allocating by Congressional district, it might be possible to see regional candidates winning a few Electoral Votes. This might make it more likely that the leader doesn't get a majority of the votes cast.
The Constitution is a document trying to anticipate the possibilities, without imposing limits on the states. It was the states that were imposing limits on the federal government, and having the federal government choose the President was designed to be a very rare case.
That's why having the state legislatures stepping in to make the call is a backstop against the federal government controlling the selection of the President.
-PJ