Problem as I see it is the negative sentiment toward Allen, not an objective look at what's best for the team.
I simply don't see any distinct advantage to having a rookie guard, playing out of position at center, new to a complicated offense, calling blocking assignments to other inexperienced players on either side of him, coming in with zero baseline rapport with the QB, as being a better option than a crippled but savvy vet.
My strong sense is that Avila at LG strengthens the OL moreso than he does at center, especially if we consider the trickledown at other positions if he starts at center.
Best for him and the team to plug him in at guard, let him play there and grow there.
If Allen's knee is indeed shot, or gets injured again and Shelton moves to center, we still have a vet calling Oline plays who has worked with Stafford. We're not moving 2 pieces around on the OL if one player gets hurt. The Oline needs to gel as a unit and learn to work together. That means leave key players in place insofar as possible.