There is even more than pride here for Bayern to play for, when looking at the rest of the teams on Group A which one out of those teams seems like the one that could become more dangerous if they were to improve during the next weeks and months? And to me Manchester United takes that spot, so instead of just letting them remain on the competition and give them the chance to improve themselves and become a dangerous rival, why not eliminate them on the spot by beating them now that they are not playing well?
I think you have a point here and that totally makes sense. It is best to eliminate them now so that they won't ever have a chance to hurt you in the later stage of the game. Things like this has happened several times in football history and Bayern Munich knows this very well and won't let it happen. Just imagine they allow Manchester United qualify and United now beings in more players by January and becomes stronger and meet Bayern in finals and eventually win Bayern in the game. So Bayern will crush them totally now.