It's clear to me that Allegri will not bench Pirlo this year, no matter how bad he's playing.
Allegri is opting for keeping the balance in the locker room and maintaining stability, instead of vastly improving the team's play with just one substitution (Pirlo for Pereyra - Marchisio as a DM).
Pirlo is a legend, one of the best of all time, and he's a very proud guy. Benching him isn't easy.
This is already very dentrimental to the team, and it will be against Dortmund.
IMO he'll continue to start no matter what till the end of the season. Then it will be up to him to realize he can't play at the highest level anymore and move to the US, or continue with Juve and play his last season. And he'll continue to start. I just can't see him being benched.